Just replace the temperature gauge. The entire assembly consists of the dash mounted dial, a long thin tube, and a sensing bulb (which goes into the intake), and there are no replaceable or adjustable components. Get a new one at Nisonger (
Nisonger Instruments :: the exclusive Smiths U.S. Warranty & Service Center since 1949) or gaugeguys (
APT Instruments International Inc). You could also send yours to Nisonger to be rebuilt, but you'll find the cost is about the same as a new one. And takes longer.
Good luck.