When I am driving, it accelerates by itself and no cruise control

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.

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

Your question is vague so I do not know if I can help you.
Try checking the cable that comes from the cruise control and mechanically pulls on the accelerator linkage under the car bonnet. It should be clamped firmly to the accelerator cable and the chain link to the throttle has to be tight if it is loose the cable can't move the throtle.
This was the cause of my cruise control not working as the clamps were loose and the cable moved rather than the throttle. Check the brake switch has the cruise control sensor wire connected securely.
Is the car rear wheel drive if so check magnets are still on the drive shaft and that the hall effect transducer if mounted close to the magnets to detect the movement of the magnets. Front wheel drive may use a transducer connected to the speedo cable or may use ignition pulse sensing if these connections fail the cruise control will stop working also. Also make sure the vacuum connection is ok to the cars vacuum piping under the bonnet.

Once you have turned your cruise control on accelerate to desired speed press SET button on right hand side of steering wheel before lifting foot from accelerator, then remove foot from accelerator. pushing RESUME button will deactivate cruise control without turning it off. Then when clear of traffic push resume again to activate cruise control. It will automatically resume to previous set speed.

It sounds to me like a problem with cruise control? Turn it off and/or unhook it, drive it a bit and see if accelerating problem goes away? If it does, you need to troubleshoot the cruise control. Any applicable trouble codes?
That sounds dangerous to me, I'd disable the cruise control till you find the problem.
I don't know what testing you have done?

open the hood and check to see that the throttle cables are not out of place, if everything looks ok then unhook the cruise control cable from the throttle body (black plastic end) by pulling up on the cable end. this will disable your cruise control from making your car accelerate at unwanted times. sounds like there may be a problem with the cruise control servo.

Go back to that VW dealer and complain. I'm sure you payed plenty, and I know in my shop, if a customer's car is not repaired to their satisfaction, I want to know, and make it right. Sometimes parts can be bad right out of the box, and sometimes problems are misdiagnosed. Go back, but don't agree to pay for anything untill you get a satisfactory explanation. Good luck.

Cruise control has nothing to do with it, your pick-up is malfunctioning, it is located below the distributor or the coil pack. Replace it ASAP