I think there is something wrong with your fan's ability to run on high speed because it should run on high speed when a/c is engaged. There is a part on the fan itself that regulates the speed of the fan. I can't recall the correct name of the part, but it can also go bad. It looks like a rectangular block of plastic.

Anybody know what part this might be? Is it a relay or something? Part of the motor? I took a quick look in there wihtout removing the cover, but couldn't see the part that Gary is referring to. I tried explaining it to the guy in the parts dept, but without seeing it myself I couldn't describe it very well.