I have a 01 Silverado with a little overv 100000 miles on it. Very occassionally, say every few months, I get hot air coming from thevents and cannot get it to turn off. If I turn the A/C on, it still blows hot. If I turn the truck off, usually for 15-30 minutes, it will return to normal. The temperature gauge indicates normal temp range when this is going on. Even with everything in OFF position I get HOT air. Any ideas? Thanks!