certainly not an expert but these are some things that might cause a heat up problem - heater coil could be dirty/plugged up and restricting the flow of water through the heater, you could have trapped air pockets in the system or as you mentioned your radiator might be dirty - also check to make sure your radiator cap is the proper one for your model of truck. they come in different "pounds of pressure" etc. I don't know about the 180 degree thermo.
