I am the proud owner of an 07 GMC sierra classic, I bought it new and have put 105,000 miles on it. On my most recent trip I put the winter cover over the grill, the temp was 10 above. As I was pulling a pretty good hill 7% grade, and it heated up, I stopped and removed the cover and the temp gage went right down. A week later I again put the cover on for a ski trip as the temp was 3 above. I again had a pass to go over, and about half way up it again heated up. Any thought out there as it has never done this before.