just started, 94 4.6 will overheat if left idling, or stop and go. It cools down once you get moving, so I think the thermostat is good, got one to replace it anyways, but haven't been home before dark to install it yet. If it isn't the thermostat what other cause could it be? Nothing has been done to the engine since 98 other than regular maintainence, and has been fine since the last radiator flush like 6 months ago. It isn't exactly warm in Virginia this time of year so it is deffinitly not the weather.