If the fans work check the pump and clean the radiator. Remember these cars are "bottom feaders".

 

The symptom is clearly due to improper signal to the sending unit, as coolant temp doesn't sproradically jump up and down between 260 and 100 degrees. I'd be willing to bet my worldly wealth, had I any to begin with, that the problem is a fault in the coolant temp sending unit, or in a wire going to and/or from the thermistor in question.

 

I had this problem solved it by tossing away the electric fan and putting a clutch fan in.

 

possibly a headgasket for the fact its staying cool well driving. but check the fan and make sure its coming on first. if it was the therostat itd be overheating well driving aswell so thats def not it.