I have a 2008 ram 2500 6.7 with full deletes and 5" exhaust and a H&S mini max tuner.
This morning I started the truck with the remote start let it warm up for 15 minutes and pulled away,it was around 23degrees this morning I had heat and the truck had good heat. I pulled in to the car wash to cancel the car wash for the winter so I figured I would get the truck washed.
When I left the car wash I got out to put my antenna back on and noticed it was starting to blow cold air,I looked at the temp guage and it dropped to 150 and then to 140 this was on the temp on the mini max!
So I drove to the express way and it still was blowing cold.
After I got up to 60 mph it got heat for a few minutes but it went away!
Then the temp guage went to 200 then down to 180 so I got off the express way and it went to 220 then to 235 but no heat! I drove over the railroad tracks and all of a sudden the temperature dropped to 180 and the heat came back I drove it home about 2 miles and checked the coolant!! It had pressure in the system but the coolant resiviour was over full it took about 2 quarts in to the radiator and I started it back up, no air bubbles or blowing the coolant out and it had good heat and not running hot!
I got on to the express way and drove for about 5 miles with no issues!
I let it sit for about 4 hours and went back out I drove it for about 4 hours round trip good heat and not over heating the temperature stayed around 190 to 198 I don't beat on the truck and I hope it's not a head gasket I'm going to put a new thermostat in tomorrow and do a block check with the dye while I'm at it! Has anybody run into this?

 

my 09 6.7 does the same except mine pumps coolant into the expansion tank as it gets warm, wont pull it back as engine cools, looking for anwsers, still looking. no bubble in the filler neck when full, no coolant in the oil, no white smoke from the exhaust when running at temp. 198-200, have changed the stat and cap, still having troubles.

 

I would try using a block check kit! It's a tube that you put this litmus fluid into then you use this bulb to suck air from the radiator into the tube,if combustion is getting into the cooling system it turns the litmus fluid from blue to yellow! But mine hasn't pushed the coolant into the resiviour bottle since it first did it!
I'm going to change the head gasket and put studs in it this spring just for piece of mind!

 

Your radiator is clogged or you have air in your lines that need to be bled out. I just went through the same thing. I replaced the thermostat 4 or 5 times water pump hoses radiator cap the whole nine yards man still pushing coolant out of the overflow ended up being the radiator was clogged.