your ecm is seeing too low, too high, or an erratic idle problem. check for vacuum leaks. your going to really have to look as there are alot of vacuum lines hidden. also give it a fuel system clean.
the idle motor is not working. when the a/c is on the idle should kick up ~100-150 rpm. your engine is going to run rough because the a/c is putting a load on the engine and the only way the engine can compensate for the extra load is by raising the idle. clean out the idle air control motor and if that doesnt work, replace it.
Thanks. I'll check it out. Somewhere I read that a rough idle can be caused by an A/C unit that is overcharged from the factory. Any thoughts on that possibility?