not sure if this helps i had the same situation without the car over heating. my rpms would randomly increase and decrease when parked or at a light. man i changed so much **** mass air flow sensor...both o2 sensors..throttle position sensor...cleaned throttle body..cleaned the idle air control valve..map sensor...fuel filter..cleaned k&n air filter..a few other things i just cant remember all....only thing i didnt check which was next was for a vacuum leak. Before that thou my brother moved my icm and BAM...that was my problem..****ing gm worked on my car once and tapped into the wires for some odd reason and left a few bare which touched upon movement.once i fixed them it solved my issue. good luck man.