I've got a 94 f150 4x4 302 AT and the same thing is happening to me. Since I don't see any real obvious vacuum lines behind the battery on this rig, I suspect that what I need to look for is a vacuum leak of some kind. I'll do that, but now I'm curious. Why is a vacuum leak causing the fan motor to cut out? I can understand the flappers in the heater box moving around, but that would/should only give me cold defrost (if that's the default position). Right?
Or is the AC actually hooked into the brain somehow and so responds to the load that is on the motor?