Providing more info on my 2003 Sienna's overheating

I submitted this question a few days ago and got so good info to confront my mechanic with. If you’ve already read this part, please skip to the end for new info.

(My 2003 Sienna minivan overheats when the AC is on. After about 10-20 minutes of running the AC, the air starts blowing warm, then it fluctuates and gets cool, but not cold. There is a humid and musty quality to the air, and then eventually the temperature gage quickly rises. When I turn off the AC the temperature quickly cools and the car runs with the gage positioned right in the middle range. I have recently had fans replaced, $450, which fixed the problem for three weeks. It started again. I was then told it needed a new radiator and thermostat. These were replaced, $500,even though I told the mechanic the car runs fine with the AC off. Again problem was fixed for two weeks. Now it’s starting again. Besides getting to the bottom of what is really wrong and fixing it, one of the things I find strange is that both times the mechanics worked on the car the symptoms temporarily stopped. If they have not addressed the problem, why would this happen, and could it be a clue as to what is wrong?)

I left out one other point, which I hope will help to pinpoint the issue. The car also eventually overheats if I turn on the heater and AC is turned off. The air coming out, is hot for a while, then begins to fluctuate and then is neutral or on the cool side, is again humid or musty smelling and then the temperature gage quickly rises. Going up hills seems to speed the overheating along with both the AC or the Heater turned on. Thanks again for your patience with a senior citizen who is not to car savvy.