6.5s have real problems with coolant pumps. There is a replacement that works better and lasts longer. Check with your dealer. You may also have a problem with temp regulator, or with heater core.
6.5 is not a bad engine, properly maintained and set up. (I have two) Both of mine have improved turbos, performance chips, many mods inside the engine itself, and will run on nearly anything you can put in the fuel tank. I run B99 often, but also #2, or #1 if it gets real cold. No additives. Both units also exhaust via 4" tubing and freeflow muffler. Newer one has a cat off an 08 Duramax, just for visual effect at emissions test station.
Last but not least, get online with the 6.5 diesel forum.
http://www.sixfiveturbodiesel.com/ There you will find many answers to 6.5 problems, and the vehicles they are installed in.
Good Luck!