Question- - - - -is that a mechanical gauge with the copper tubing from the cylinder head to the dash cluster? If so, check the ground path from the engine to the body/frame assembly. I've seen engines with new motor mounts and a poor ground path because of a missing ground strap or painted surfaces interfering with the current flow try to use the temp gauge capillary tube as a starter ground. It usually gets red hot and melts when that happens.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!