My check engine light comes on intermittently and my cruise control drop out while driving.

Check to see if your third break light is working, usually cruise control and third break light are on the same circuit. Bad gas mileage could be numerous. Does it smell like raw gas out the exhaust? Could be O2 sensor going bad.

It seems the pcm is detected a fault with the engine. Even if the problem goes away , the vehicle will have to go three drive cycles without engine faults before the cruise control use will be allowed by pcm.

Does your RPM gauge only go to 1000 and not record anything higher? As far as your check engine...on the Subaru Legacy the check engine usually comes on when there is an issue with something within the emission control sensors...and typically it is an issue with the forward O2 sensor going into the Catalytic Convertor. As a safety feature, the vehicle's cruise control will not work when the check engine light is engaged and the light will flash intermittently. The repair of the forward O2 (oxygen) sensor should run about $150 - $200 to replace the sensor. As with the case with most sensors, you may not notice any performance difference in the vehicle operation but you should have it diagnosed and repaired when possible because overtime the operation can be impacted.

hi i had this problem with engine management light and no power over 2000 revs the solution was replacing split breather pipe and all codes checked and cleared, the cost was £30 @ jagtech stockton hope this helps

cruise control operation is reliant on the following
road speed sensor ( speedo or VSS sensor )
engine rpms tacho/ ignition coil/module problem
clutch pedal sensor (VSS in automatic )
gear selection position sensor ( not properly adjusted causing flickering of the selector light)
brake pedal sensor
bad ground wire connections
r all need to work together or the CC will stop working
nothing to do with vents and all that you mentioned.
wire connections and harness connections for the transmission are suspect
CC will not operate under certain engine rpms and road speed

check your cruise control unit first

it might be your vehicle speed sensor. It affects your speedometer, your cruise control and your trans shift points. Also, your throttle position sensor (TPS) affects shift points.

possably a vacuum leak. the check engine light comes on with an emmissions issue, ie vac leak, and it will not allow the cruise to work. once the error is fixed, your cruise should work properly again

sounds like the brake light switch is the culprit for both the intermittent brake lights and cruise control problems. the engine warning light could be an unrelated issue that needs to be diagnosed.

See if you can find out if there is a code for the check engine lite and the abs, those can be related if the speed sensor located on the top of the rear axle is intermittantly failing. That sensor sends the speed signal to the pcm and the abs controler witch will also turn off the cruise control.