Thermostat

What's the temp when sitting at idle? How far does it drop when you're driving?
The thermostat wouldn't cause a symptom like this. You need to first check that your radiator fans are working.
When you are idling, the radiator depends on the fans to draw in air and cool the system. Then when you are driving, the grill sucks in all the necessary air.
Also, as the engine revs higher (when you're driving), the water pump spins faster, coolant circulation increases and the pressure in the system builds. All of these bring the temp down slightly compared to when you're sitting at idle.