AC problem in 1995 Firebird Formula?

I own a 1995 Firebird Formula convertible. I just purchased it a few months ago. Up until recently, the air conditioning was functioning. Then out of the blue about 2 weeks ago, the air stopped being cool and I stopped hearing the compressor kick on. Ok, move ahead to last Friday, 6/20. I get my oil changed at Jiffy Lube. I also have the serpentine belt changed. The last thing they did was a free AC check as I told them it wasn't functioning and I just wanted to know why. They come and tell me that it is, in fact, working. I come out to feel cool air coming out (they said 20 degrees cooler) and I hear the compressor kicking on and off. Don't know why it started working again, but it did, until... yesterday, Sunday, 6/22. I left to run an errand and turned on the air. Compressor kicked on and I had cool air. After going about 1 1/2 miles, I started getting nothing but warm air and the compressor hasn't kicked on again since. Any suggestions?