Lots can go wrong with the pump. If the impeller is pushed too far onto the shaft, it will not have the correct block pressure and have a slow steady overheat under a load. Make sure that the seal around the stat is in good condition or, there at all. If it isn't there, coolant will go around the stat and the stat won't open fully, and go thru the bypass loop, again resulting in a steady overheat loaded. DO NOT run a diesel without a stat. They have two cooling loops, bypass and radiator. With the stat closed, the coolant comes out of the intake and flows out the rear of the stat into the bypass loop back to the pump. With the stat open, the end of the stat seals against the front of the stat housing, sealing the bypass loop and forcing the hot coolant through the radiator...hence radiator loop. If you run without the stat, the coolant will take the path of least resistance...back into the pump via the bypass. It leads to a steadsy overheat loaded too.
Taylordc, you took the fan off. Make sure the fan is on the correct way. Also, IMHO, you need to verify the accuracy of your gauge with a known good one. The IR thermometer is a good way to do this. Also, have the cap checked. If the seal is damaged or not there, the coolant will boil close to 212 'cause the cap isn't working as it should by raising the air to boil temp.
Keep us all up to speed on what you find. These types of issues are sucky for the owner, but can help others when they have similar problems.