Sorry to dredge this up from the dead, but I may have finally found the silver bullet that fixes this problem.

Despite my earlier post of success, the gauge started jumping around about a month after I posted that, and I'd just lived with it until now, since I'm back down in a warmer part of the country I'd like to be able to trust the gauge.

I did some fooling around with the truck 100% cold. I'd turn the key to 'on' and the gauge would come right up to the middle. The signal connection was good, the IGN power connection was good, the ground was kind of sketchy.

I put a star washer on the middle mounting screw of the temp gauge (the one that carries the ground connection) so that it really dug into the copper contact and the screw, and it's been doing fine ever since.

I think the smooth nature of the contact and the washer on the factory screw just lends itself to poor conductivity after 10 years or so.

I'll keep an eye on it, but I think this may do the trick. Sure would be nice to not have to buy 300 bucks worth of new factory gauges.