I have also observed this behavior on all of the G bodies that I have owned over the years… I think of the 12 V supply in these cars as having a primary and secondary region. The primary region is under the hood and basically consists of the alternator battery and wiring between the alternator, battery and starter. And let’s not forget about the grounds under the hood. The secondary part of the system is the passenger compartment, and it is fed from the positive supply at the starter through the fusible links into the fuse block. In the past when I’ve observed this behavior, I have also measured the 14 V at the battery While the car is running. At the alternator, the 14 Volts does not show the fluctuations that the passenger compartment shows. I’ve never really analyze it any further than this but I have observed that the supply and grounding is beefier under the hood than it is into the passenger compartment or secondary region.
all of the above, + bad circuit board or contacts on the cluster/gauges, cluster not pushed in all the way, lots of things could be going on. Take it apart and put it back together again, make sure its all snug and tight. if its all acting flaky that's where I would start.