Hi all. I'm in some serious need of some help. I picked up a 2007 GMC W4500 boxtruck (Deisel) and having an overheating problem. When driving the needle goes back and forth from about half to overheating. When its around half if I turn the heat on it works for a bit then starts getting colder. At the same time that it is getting colder the needle is rising. I can turn the heat off and the needle will lower. Regardless the needle at some point will continue up to over heat. When it does get top overheating I am also losing power. Thanks and any help will be greatly appreciated. As an FYI I did change out both thermostats. And I also noticed that the reservoir has the same fluid that was always in there and has not changed.