Twice in the past two months I've gotten my a/c charged and twice has it seemed to work. I'd drive about 25 miles and the very nice, very cold air turns very warm. My thoughts after the first instance were that I simply needed to replace some o-rings, before I spared the expenses of the rings, however, I thought it best to double check that I didn't actually have a leak and sure enough after vacuuming the system it held for at least 30 min. No leaks detected. So we charge the system and I leave to go elsewhere a short while later comes the warm/hot air. Upon further inspection I notice my compressor clutch isn't engaging, it appears it hasn't worked in quite some time. I haven't checked relays yet however I'm confused as to why the system is cold and half an hour later It'* warm again without leaks. Do I replace the clutch assembly, the compressor, or is it electrical?

1991 Pontiac Sunbird LE 2.0L

Sounds like your compressor is on its last leg, I'd change it.