Thanks for the replies. I called my dealer and I'm still under warranty (by less than 60 days, time to buy an extended). I was only able to speak to a service rep who had no idea what I was talking about and didn't seem interested in anything I had to say about the problem. I have an appointment for Monday morning. I'm sure it's the evap temp gauge. Hopefully what info you all helped me with will cut down on the time the car is in the shop. My dealer is just far enough away to make it a major hassle. But at least it's under warranty.
One last thing. Tonight I had the normal problem. A/C cut out soon after ignition. But later when I was driving home I monitored the evap temp while driving. It still bounced around from 40 to 90 very erratically, but never went over 100. The A/C didn't cut out the whole way home (about 20 minutes). That's as long as the compressor has run in weeks. The two times I monitored the evap temp the compressor only cut out when the gauge jumped over 150. That still seems to be the source of the problem.