1999 Pontiac Grand Prix Car Running Hot while parked and ru

I have a 99 Grand prix GT 3800 series 2 v6. It has a new thermostat 180 degree and all teh hoses are good. There is no leaks in the cooling system at all. The problem is that when I am parked in a parking lot with the car running the car starts to get warm. I have a feeling it is the cooling fan. I am pretty good at mechanical stuff myself but am I thinking logically that this is most likely it. The reason i say this is when i start driving again the car runs just fine and starts cooling donw again.