Any problem on these cars isn't easy
I would run a fresh ground wire if you haven't done so already. I feel as if these cars should have come with an extra ground cable for the instrument cluster. A wonky instrument cluster was one of the principle reasons why my stepdad put my car into storage.
There are countless threads on how to do it, but redundancy is a good thing.
Get a good gauge wire and some spade plugs. Latch the spade ends onto the rear screw on the cluster that is on the lower right of the spedometer if you're looking at the rear of the unit.
Then connect the wire to the ground collection to the chasis on the other end.
Good luck! Let us know if that was the case.