Air overheating shop says needs Radiator

Just wondering what all you experts think? THey kept my car all day yesterday and said the reason it is overheating is the radiator needs to be replaced. I had it flushed twice and they even took it out and tried to flush it again but I am leary of replacing it because the car doesn't overheat except when the air is on and at night no problems at all.

I have $900 in this air fix already and if the radiator doesn't solve the problem I am out more money. I told him I would just live without air for the summer.

replaced compressor 7-1-03
replaced belt 7-2-03
repaced tension belt adjustor 7-15-03
replaced thermostat 7-16-03 (it was bad)
replaced thermostat again 7-16-03 (it was ok when taken out)

Help you wonderful experts!!!! Thanks so much