2000 Grand Voyager overheating with AC is on

I have a 2000 Plymouth Grand Voyager that has recently started overheating when the AC is on. If I never turn on the AC, it operates normally. If it does overheat, I can switch the AC to HEAT and it cools the engine down. So far, I have: replaced the thermostat (eventually just removed), replaced the serpentine belt, replaced the water pump, replaced the temperature sending unit, replaced the radiator cap. The radiator is relatively new (less than a year old). I have a new radiator cap on it with a thermometer. Normal operating temp (according to the radiator cap) is about 190.. When the dash temp gauge says it is overheating, the radiator cap still says everything is normal. But I know the dash temp gauge will only say overheating of my AC is on. The AC eventually stops blowing cool, the engine temp gauge then starts heating up, I switch to heat, and the engine temp gauge cools down.. The radiator cap temp stays the same the whole time..
????? Please help, really need to figure this out