I have a overheating issue with my 93 gs. It only overheats on the highway but once I turn on the heat in my car the temp drops down to normal. Is this an issue with the thermostat?

 

I would replace the thermostat and radiator cap first, and add water while the cars running and try to see if theres any bubbles in the system you can get out. If there proceeds to be a problem.. Is your car puking excessive coolant in the reservoir (is it bubbling?) It could be a blown head gasket but that is less likely, but a possibility. Another less likely cause is a plugged up radiator, You could feel the radiator and see if there are any "cold" spots in it when your warming up the car.

 

What maintenance has been done to the cooling system recently? If you don't know or are uncertain, that's a good starting point. GL.