[joeydurango]

Okay, final update for now. Looks like several of you were correct, and that the problem is in the gauge itself. I have power to the gauge. I have a solid ground. And as of today, I know that I have the same distributor signal to the gauge, whether it's working (warm) or not (cold). So, whatever broken connection is expanding and making contact with heat is indeed inside the tach, probably on the circuit board Ray mentioned above.

Since it works once warm, I will leave it alone for now. If it gets worse, I may try to disassemble the gauge and repair it, or just replace it. I dislike threads with no resolution, so I figured I'd post a final word!

Tom, since you subscribed for info, I'll say that my tach does what you've described for a short period of time between it not working at all (cold) and working perfectly (warm). When it's in between, it only registers the higher RPMs. Given what I've learned here it sounds like yours may have a connection that is tenuous, and that doesn't change with temperature, unlike mine. But to be sure you could do what I did and check power, ground, and signal to the back of the gauge with a multimeter. Good luck!