What I'm trying to do is isolate the gauge cluster from the vehicle completely. This eliminates the car and will help isolate your problem to the cluster only.
You have already eliminated the wiring, the gauges and the grounds. Not much left is there LOL !!
BTW, I noticed that you didn't say if you had connected a ground from the metal dash to the vehicle itself. Should not be a problem as the dash is bolted to the car body but you never know!