86 Pontiac 6000 STE overheating when sitting sit

86 Ponitiac 6000STE 92,000mi - have flushed and refilled the cooling system - it is full and new - but when stopped in traffic or the motor is on but not moving, the engine starts to overheat and the ac turns off - once you start driving again, the problem goes away and teh engine temp returns to normal - it seems the motor rpms go down in this situation - normal prm is 7-8000 - is there anything I can check on the engine or do to stop this problem - I saw one comment about cleaning the fuel injector - but how do you do that and where can I learn how to do it myself....