Originally Posted by Dreadnaught

Could be a possible intermittent fan issue. The fan should be on when you turn the AC on to help the engine stay cool due to the extra load of the AC compressor being engaged. The fans also come on at lower speeds and at a stop to help keep the engine cool since not as much air is going through the engine bay. The fans also come on when temps get to high even though you are driving and the air going through the engine bay. I

You could take it to the dealer and they should be able to monitor desired fan speed and actual fan speed to see if there is any variance. You could also see if there are any stored codes or any for overheating.

Is your check engine light on?