I finally decided to spend the money on a newer truck to baby my obs even more and I and get rid of my beater cummins. I went to a dealership with a check in hand to buy an 05 f350 lariat 4x4 cc lb that was super clean and on my last test drive i put the heat on its highest setting and even with the truck warmed up it blew cold air but when I stepped on it it would blow hot air till i let off, then cold again. I mentioned it to the salesman and he didnt seem eager to fix it but he finally said he would address the situation. I backed out of the deal and told him to call if they fix it then id be interested again. I had the impression that it was egr related or a head gasket assuming the worse. the truck showed signs of past work on the heads and having been around the 6.0s for years now i wasnt at all suprised. just wonderin what u guys thought it might be.