Thermastat is sticking an the clutch fans along with good speed is keeping it from blowing up I'm guessing at its worst when you stop you hear bubbling sound? If not just a matter of time. Get the therm replaced an make sure u get the factory replacement someone could of put a hotter therm in order to get better heat in winter or the mods your doing are causing the extra heat next MOD would be a bumped up intercooler along with the water pump that's my answer hope it helps.

If the fan is properly working you may want to replace the coolant sensor for the computer, it’s not the one for your gauge or temperature warning light this one is the computer’s temperature sensor that the engine uses when your car is up to temperature and the computer will switch to a closed loop that helps the computer to fine tune the fuel mixture, if this sensor is not operating properly the engine will never go to a closed loop status and you will notice your fuel mileage drop noticeable.