This usually happens in the spring and fall, when I may use the heater in the morning and the AC in the afternoon. I'll try to use the AC, and it will start blowing VERY hot air. I can turn the fan down to reduce the amount of air. But, turning the thermostat or turning on/off the AC has no effect. Sometimes, it switches when I bang the top of the dash or maybe it was coincidence.

Over the years, the dealership has tried replacing one or two things, but it was really too much trouble (or too expensive) to keep taking it in. They really didn't know what the problem was...and, of course, it worked fine when they looked at it.

Any ideas or suggestions?