Quote Originally Posted by rspears View Post
I'm thinking an intermittent ground problem, a battery going to ground or an alternator getting crazy.
I'd be looking/checking ground, it is the common link, and would be easy to check. Two alligator clips and a some wire, not knowing how the instrument cluster sits. Just throwing out the idea you figure out the attachment point. Jumping all over says to me intermittent connection.

You could clip on the mounting bracket of any one of the gauges, again the common link to all, go to a different body bolt beyond the dash. Or to make sure you eliminate all the ground paths from the body go straight to the bat -, then go for a test run, I think you said the threshold was 70, see if it repeats, or is stable. Then remove the giant ground jumper you have and have another run at it. If this is a glass body then I would think you have a ground buss that all the gages tie to, it that is the case attach your jumper there.