My truck runs nice and cold when I'm driving but if I let it idle for ten minutes or so, it overheats.

i have changed the thermostat, replaced the fan clutch, put new belts on it, flushed the radiator, and pressure washed all the dirt out of the radiator as well. What else could be causing this problem?

 

What about the water pump?

 

no no I didn't change the water pump. I assumed if it was the water pump it would overheat while moving and while idling.

 

Don't forget you have more RAM air going through the radiator when moving as compared to sitting still.
Also, if the impeller is worn, higher RPM produces greater flow.

 

Overheating at idle is usually caused by an airflow problem. Start looking at the fan clutch, a missing shroud, damaged fins on the radiator, plugged fins on radiator and/or a/c condenser if you have one. Even though it is new, there are some low quality aftermarket fan clutch units out there, I would still look closely at that fan clutch when it is hot to make sure it is not free wheeling. I would think a water pump issue would also give you more problems at highway speeds with reduced water flow where the engine is putting more demand on the cooling system at higher engine speeds.. Might also be a good idea to get your cooling system pressure tested too. May sound silly but make sure the stat isn't installed backwards.Just trying to cover all bases.