Unexpected acceleration on my 2003 montana with cruise control

You do not have a cruze module like the old days. The throttle body is an electric motor and is controlled by the ECU

if it just started to happen you may have lost a magnet off the drive shaft. If it is a front wheel drive then you would have two magnets on the flywheel if you lost one magnet it would cause problems.
The other possible cause if the sensitivity setting is set too sensitive then the cruise control will surge .
Setting it lower will allow it to slowly respond to speed changes but if going up hill it will take a while to increase the acceleration . Or going down hill it could accelerate too much before the cruise control realises it is going downhill
Also a very wet road can cause the wheels to spin and cause the cruise control to lose the plot and accelerate to dangerous speeds.
I have had this happen to myself once so be wary of cruise control use in wet conditions.

Cruise control only maintains speed by utilizing the accelerator. To speed up it increases it and during slowing is removes it. The cruise control will not apply brakes in the event of a steep downhill run. In this case the vehicle will start to accelerate in the same manner as if the cruise control was not engaged. But in theses cases, most cruise controls, once the speed exceeds a certain amount above the set speed, will auto-disengage as a safety feature.

It is HVAC 1fuse #39 in the rear fuse block ,this powers the on/off switch for cruise control ! Fuse Block - Rear Beneath the left rear seat. What engine does it have ? 4.2 L or 5.3 l There is no vacuum involved in the operation of the cruise control ! Your vehicle has electronic throttle control .
Cruise control is a speed control system that maintains a desired vehicle speed under normal driving conditions at vehicle speeds above 40 km/h (25 mph). Steep grades may cause variations in the selected vehicle speeds.
The following are the main components of the Cruise Control System:
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The powertrain control module (PCM)

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The On/Off switch

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The Resume/Accel switch

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The Set/Coast switch

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The TCC brake switch

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The stop lamp switch

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The throttle actuator control (TAC) motor

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The vehicle speed sensor

Cruise Control Engaged
The cruise control system will engage and adjust the vehicle speed based on the activation of the following cruise control switches:
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On/Off

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Resume/Accel.

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Set/Coast

The cruise control switches are located on the multifunction/turn signal lever.
The powertrain control module (PCM) monitors the signal circuits of the cruise control switches in order to determine when to capture and maintain the selected vehicle speed. The PCM uses the throttle actuator control (TAC) module motor in order to control the vehicle speed. For further information on the TAC System, refer to Throttle Actuator Control (TAC) System Description in Engine Controls - 4.2 L.
Ignition positive voltage is supplied from the 10 ampere HVAC 1 fuse to the cruise control switch via the ignition 3 voltage circuit. When the normally open cruise control On/Off switch is turned On, the switch closes and the PCM detects a high signal voltage on the cruise control on switch signal circuit. When the normally open Set/Coast switch is pressed, the switch closes and the PCM detects a high signal voltage on the cruise control set/coast switch signal circuit. To engage the cruise control system, turn the On/Off switch On and momentarily press the Set/Coast switch. The PCM will confirm that the cruise control enable criteria has been achieved. The PCM will engage the cruise control system and record the selected vehicle speed. The PCM sends a class 2 message to the instrument panel cluster in order to illuminate the cruise control indicator. Pressing the accelerator pedal, while the cruise control system is engaged, will allow the driver to override the cruise control system in order to accelerate the vehicle beyond the current set vehicle speed. When the accelerator pedal is released, the vehicle will decelerate and resume the current set vehicle speed. The driver can also override the current set vehicle speed via the Set/Coast switch and the Resume/Accel switch. When the cruise control system is engaged, pressing and holding the Set/Coast switch will allow the vehicle to decelerate from the current set vehicle speed without deactivating the cruise control system. When the Set/Coast switch is released, the PCM will record the vehicle speed and maintain the vehicle speed as the new set vehicle speed. When the cruise control system is engaged, momentarily pressing the Set/Coast switch will allow the vehicle to decelerate at 1.6 km/h (1 mph) increments for each time that the Set/Coast is momentarily pressed, with a minimum vehicle speed of 37 km/h (23 mph). When the normally open Resume/Accel switch is activated, the switch closes and the PCM detects a high signal voltage on the cruise control resume/accel switch signal circuit. Activating and holding the Resume/Accel switch, when the cruise control system is engaged, will allow the vehicle to accelerate to a greater vehicle speed than the current set vehicle speed. When the Resume/Accel switch is released, the PCM will record the vehicle speed and maintain the vehicle speed as the new set vehicle speed. When the cruise control system is engaged, momentarily activating the Resume/Accel switch will allow the vehicle to accelerate at 1.6 km/h (1 mph) increments for each time that the Resume/Accel switch is momentarily activated, with the maximum acceleration total of 16 km/h (10 mph) over the current set vehicle speed . Momentarily activating the Resume/Accel switch, after the cruise control system has been disengaged by pressing the brake pedal, will recall the previous set vehicle speed that is recorded in the PCM.

suggest checking cable on accelerator throttle arm in engine bay where it connects from cruise control to accelerator throttle arm on engine. Thinking if it is loose it may get looser when hot.
Check connectors on electronics in engine bay on cruise control.
Check sensor for speed. Is it ignition maybe a scotch lock connector that expands with heat and cruise control looses sensor for igition pulses. How does it sense speed ?
Is it magnets on the drive shaft maybe the Hall efect transducer is close to exhaust pipe under car and is being affected by heat.
Is it front wheel drive ? check for a pulse sender on speedo cable in engine bay maybe this is getting cold.

Check cruise control cable is oiled adequately in cable sleeve .

check vacuum connector is ok in engine bay also ?

Cruise Control
Any given speed above approximately 22 mph (35 km/h) can be maintained with the cruise control by operating the lever.
-see photo

1. Accelerate and set: Lift lever briefly to set speed. Hold lever up to accelerate.
2. Decelerate and set:
   Depress lever briefly to set speed. Hold lever down to decelerate.
   Normally the vehicle is accelerated to the desired speed with the accelerator. Speed is set by briefly pushing
   the lever to position 1 or 2. The accelerator can be released. The speed can be increased (e.g. for passing)
   by using the accelerator. As soon as the accelerator is released, the previously set speed will be resumed automatically. If a set speed is to be increased or decreased slightly, e.g. to adapt to the traffic flow, hold lever in position 1 or 2 until the desired speed is reached, or briefly tip the control lever in the appropriate direction for increases or decreases in 0.6 mph (1 km/h) increments. When the lever is released, the newly set speed remains.
3. Canceling

To cancel the cruise control, briefly push lever to position 3.
When you step on the brake or the vehicle speed drops below approx. 22 rnph (35 km/h), for example when driving upgrade, the cruise control will be canceled.
4. Resume
If the lever is briefly pushed to position 4 when driving at a speed exceeding approx. 22 mph (35 km/h), the vehicle resumes the speed which was set prior to the cancellation of the cruise control. The last memorized speed is canceled when the key in the steering lock is turned to position 1 or 0.
Note:
If the engine does not brake the vehicle sufficiently while driving on a downgrade, the speed you set on the cruise control may be exceeded and you may have to step on the brake pedal to slow down.
As soon as the grade eases, the cruise controlled speed will again be maintained as long as the brakes were not previously applied, or the lever may be used to resume the previously set speed if the brakes were applied.


http://www.benzworld.org/forums/attachments/w202-c36-c43/153145d1197313761-pdf-owners-manual-w202-1995-2000-1999-c280_owners_manual.pdf

Check the actuator under the hood. Has a cable that goes to the throttle. Is it plugged in? I believe there is a vacuum line to it also, is it good?

check the rpm and road speed when this happens . The cruise control will increase to keep RPM constant but as the road speed drops the auto may be shifting down and as the rpm increase it shifts back before the rpm adjusts. You can override a cruise control in this situation by simply pressing down on the accelerator to maintain the power setting and road speed and when the situation is passed and you take your foot of the accelerator the cruise control will return the system to the set position without you needing to do anything

There are numerous reasons why the cruise control would stop working. Could be a circuit board, the cruise control module, a vacuum leak, fuse, or something else. You'll have to take it to a shop.

I just bought a 2011 Sonata and am experiencing the same problem. Going downhill with cruise set at 55 mph; speed increased to 70 mph and was still accelerating (from the downhilll grade) when I finally pressed on the brake.
This is not normal. My Honda Accord never varied from the cruise setting by more than 3 or 4 mph ... going uphill or downhill.