Ok I will have to give more thought to why this is happening. BUT rest assure that the problem is NOT related to the underdrive pulley in either way. Remember ONLY the a/c compressor is operated by a belt, and doesn't affect the blowing of air in anyway. So if this problem was because of the pulley it would only affect your cold air, not the symptoms you have. There is a relay for the blower motor however, it is near the fuse box inside the car, not the one inside the engine bay. Try troubleshooting the electrical system, see if power is going to the fan when you have this problem, that will tell you if it's in the fan or before the fan.