Any resolution to this?
I have the exact same problem. AC warms up. Engine overheats. I replaced the thermostat on the advise of a coworker, and found that the old one had cracked in the housing! I thought I had the problem conquered!

Not so fast.

Seemed to help a little but the car still heated up in stop and go traffic. When I got home, I left it running in the driveway and let the temp climb, which it did almost into the red. Then fan finally kicked on and brought the temp down very quickly.

It seems to be a problem with the fan, but what would cause it to wait so long to turn on. The relay, bad contacts, a temperature sensor, the (gulp) ECU...? Any help would be really cool (pun).

 

Problem solved: relay was bad. AC and engine both working as they should.