After being parked overnight, today I drove about 3 miles for a quick pickup and the air conditioner worked fine. After parking and turning off the car for about 20 minutes, I restarted to drive back home. this time the Vents only blew warm air. After being parked again for about 4 hours, I drove my car for a couple of blocks with the air conditioner on again, this time it was cold air again. This same thing has happened numerous times over the last two weeks. It seems after the car is not running for an hour or more, the air conditioning will work fine for a while. I have made no longer trips recently, to test out how long it will keep cooling. Most of my trips are short.
My guess is a failing compressor clutch coil. Get your mechanic to check for voltage and ground at the compressor field coil when it is not working. This will tell if that is the problem. If battery voltage and ground are present with no compressor operation then the clutch field coil is defective. To confirm bump clutch plate, with a piece of rubber hose ect, while engine running with AC on see if it starts working, bet it does!