2000 Ford F350 7.3 l over heats

Truck starts good, no excess smoke, runs good. I havn't pulled a trailer yet to see if it over heats under a load but I do think its going to. I have run it hard in 4 wheel drive in deep snow and the temp shot right up to the red. The heater part of the problem has got me thinking of an air bubble stopping water flow.
I pressure checked the system and found no pressure drop. The problem with the heat seems to get worse the colder the outside air temp is.

Ok, I did the temp check to day. When the temp go's up so does the reading on the inferred gun. But it cools down very fast. Just about the time it takes to pull the truck over and get the hood open its back down to operating temp. When the heater blows hot the air from the dash vent its 115 deg's. When its worm air its around 70. Now this AM I drove it 25 miles didn't get any heat until I drove appx. 15 miles engine water temp about 94 deg. Shut down for about 45 min. On the return trip had worm heat right away. When I had temp gauge in the red zone I didn't get any real hot air from the dash. Haven't lost any coolant. When I drained the system I got 6 gals. That's just what I was able to put back in it when I refilled the rad.

Rad's clean. When I put the new water pump on I cleaned both rad and a/c condencer. Today it was 50 and the frist run I had no heat, Ran it up to 75 mph and engine temp went right up. Still no heat. Shut it down for about 1/2 hr. On the second trip had heat, worm but it was heat. Ran it up to 70 mph engine temp went up heater temp went down?
I think I'll hang a for sale sign in the window!