Intermittent air problem

[eggplant]

The compressor on my 2003 Celica seems to shut off intermittently and the AC light flashes, then the air gets hot. If I turn everything off and then back on sometimes it starts working again, sometimes not. When it's working it's plenty cool, when it's not it's compleatly dead and just blows hot air.
I took it to the dealer but unfortunatly that was one of its good days and after 2 1/2 hours they could find nothing wrong with it, the thing had plenty of Freon. The next day the problem surfaced again. I'm hoping somebody here is smarter than the Toyota mechanic.