Hi Bill, this probably won't be much help, but I had something similar happen on our Commander when I was trying to hook up the lights for the new electric tachometers I installed. When I hooked the ground up, the instruments went crazy. I checked between the instrument ground and engine block with a digital volt meter, and found about 6 volts. It should have been zero volts.
Sometimes ground systems get some circulating currents in them that can cause problems. I didn't take the time to trace down the problem because the old ground wire was in a place where I really couldn't reach it. I just dragged in a new ground wire from a spot where I knew I had a good ground, and that fixed the problem.
So, you might try fishing in a new ground that you know is good, and see if that fixes the problem.
Best wishes, Curt....
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1967 fiberglass 38' Chris Craft Commander Sportfisher with twin 427 CID 300 HP engines.
