67 Barracuda instrument voltage conundrum

Yeah, it's hard to imagine a grounding issue that would be that stable. The gauges are dead stable. I could see a ground causing erratic readings, certainly. My plan is to pull the cluster and swap the fuel gauges. I do have an external IVR module, but I am not eager to cut into a functioning gauge... my soldering skills are demonstrably mediocre. While I have the gauges side by side I will see if I can compare readings. I have a DC power supply I was using for testing.