1998 Ford Ranger

I have a 1998 Ranger XLT 2x4 4.0 V6 with 140,000 miles. While driving at highway speeds the engine temperature gauge spikes and claims to be reading the engine as running hot. By turning on the heat inside the cab at full blast the gauge settles back to normal. An oil change is due up (i'm 500miles past the mileage) but I have a hunch that its my thermostat. Am I close or am I barking up the wrong tree?
May 4, 2007.

What happens if you let it warm up parked? Does it overheat then? Usually if an engine overheats at highway speeds. You have a circulation problem, or a pressure problem, Have the sysem and the radiator cap pressure tested. If the system does not hold required pressure, it will overheat.

May 4, 2007.