There's a capacitor that triggers the compressor. That's what usually goes (if you're lucky, 'cause it's relatively cheap). Mine is acting up too. Called a 24hr place, but they were booked solid. Overnight, my compressor started working again - so I scheduled a routine maintenence for Fri, since they said they had a lot of people with no AC at all. Naturally, the compressor stopped again around 3pm, at the height of the outdoor temp.