!992 400SE overheating, HELP!!!!

My 92 400SE oerheated once last year, I replaced the cap, refilled the radiator, and it run ok until now, it was about 90 degrees out and I was running the AC, and it overheated again, what am I to do?Why is it overheating? It only has 120,000 miles, and it is all well maintained.
What should I look for?Ideas please, this is really bugging me!!!!!!!

Thanks to you all