Tach problem

Here are the important numbers. 91 xj 4.0ho no codes present.
The problem: The tach will quit working until I shut it off and let it sit for a short time (10 min). Then it works fine. Typically it quits after 10-20 miles on the ride home and won't start again until I stop for gas later on the drive or when I fire up the next morning to go to work. Any ideas?

Bad ground, either the tach in the instrument cluster, or the cluster to the chassis?

Ok, so where are those grounds? I've never had the cluster out but anything is possible.