GM-GMC: coolant trouble, chevy silverado, coolant temp

Question
i have a 2001 chevy silverado with a 4.8 motor.but i've noticed when i drive the truck the coolant temp stays normal but whin the truck stops or sits just running the temp gets hot and it starts to overheat i checked the coolant level and all the hoses everything good and i've asked and got water pump or thermostat could be so what is causing it?

Answer
could be the thermostat but i had this happen to me in a car and it was because of the vaccum wasnt strong enough to put enough air in while idling to cool the engine. check the engine fan and see if its on when its hot or if it stays off or doesnt come on.. i had to put a bigger fan with a toggle switch and turn it on while my vehicle was idling.

check your air filter it maybe needs changing and the air intake