I am having a problem with my truck as of late. Started bout two days ago, the heater would blow hot air than cool air. There was no rhyme nor reason, did it maybe three times over three days. Than today after work I was sitting idle at a red light and my "chaeck gages" light went off. Mt temp was pegged hot, I pulled over and let it cool for a bit. I started heading down the road again and noticed if I was at a high RPM with the heater on I was in great shape, lil lower than half. But at a stop the temp starts to climb and the heater blows cold. I put it in nuetral and tacked the RPM's up a bit and hot air blows from the heater and the temp starts to lower again. I am assumeing that it is the T-stat, but I called the dealer and they are telling me it sounds like an alternator problem; altinator controls the coolant fan. I know this is true in some GM models, but the temp variable at the heater vent is not making any sence to me. Any ideas or suggestions?