The engine in my 1996 Windstar 3.8 is running cool. I drive 30 miles to work one way, and by the time I get there the temp gage barely moves. I know the engine is runnning cool because it runs rougher when this happens. If I drive through slower traffic for awhile it will reach more towards normal operating range. I have already replaced the thermostat thinking it was stuck open but no luck... any ideas???