Mehran22,
Get your refrigerant levels checked up. I.e. high and low pressures, under the sky, between the hours of 11 and 1 pm. According to the ambient temp, the H and L pressure should indicate a certain level. Your AC tech would know what that is.
Get those checked up first, then get back to us. There could be a leak somewhere. Or you could simply be low on gas/freon/refrigerant. But then again, why did it get low? Aha! Get the pressures checked up.
Side info: what happens is, when the compressor get's too hot, it will trip in order to conserve itself and the engine. You know, to keep it from further wear and tear. You might also want to get your compressor's clutch relay checked up, if the pressures check out fine.