My 2004 grd prix gtp is hot when still and cools off when driving

Check your rad fluid level again. The first time you checked it, did you also top it up? If so there may be air in the coolant lines, either from the hoses on the rad or heater blower hoses. This can be caused when pump and thermostat was replaced. When the engine is cool, take the rad cap off, fill the fluid level, check that your overflow is at the cold level and start the car. You may need a catch basin under the bottom of the rad, below filler cap to catch overflow rad fluid. As the engine warms, the fan should kick in and thermostat should open causing the fluid to exchange into the heater blower. If there is air in the line the air will appear in your rad as just that, air. The fluid level will drop. With the engine still running, Manually top up the rad again until full and wait for the fan and thermostat to open again. This time you should not need top up. Turn the car off and let cool. Once cool,check the rad fluid in the rad is full and the overflow level is at cold. This may solve your problem. Let me know how it goes.

Probrally the thermostat or water pump