BrickoCheese has ID'd the most likely cause. I've been tempted for some time to buy one of those little infrared temperature detectors for occasions like this - just aim it at the thermostat housing, pull the trigger, Bingo- there's your temperature. Anyhow, a couple of possibilities for your symptoms: you seem to get overheating only when the engine is working (not at idle), so it could be that your radiator is scaled up and can't transfer enough heat energy to the air. When you turn on your heater (it's just a mini-radiator) it makes up the difference. Another possibility is your lower rad hose - the one that feeds the water pump's inlet (ie: the suction side of the pump). If that hose is old and soft it may collapse under the suction of the pump at higher RPM and restrict coolant flow. Been known to happen. If it feels soft, then replace it as a normal maintenance item.