Erratic Gauges

Hello Folks,

I have a funky gauge issue on a 70 charger that I am hoping to get some insight on.

So the car has brand new wiring and a new instrument cluster circuit board. All gauges worked as well as these factory gauges do but a couple months ago I removed the cluster to add in this gauge light surround to the lens to keep the dash lights from bleeding around the gauges and blinding you at night. In the process I completely disassembled the cluster to clean the housing and put in new blue light lenses. I reassembled the cluster and added in a ground wire that went from a screw on the wiper switch to a couple of the hold down screws that ground the circuit board to the cluster housing. that wire then went to one of the hold down screws for the tic toc tac. I then had a wire from from that same tic toc tac screw go to a screw on the underside of the dash.

When I installed the cluster the dash lights work, tac works, fuel gauge works but the oil and temp gauges continually bounce. According to the FSM it says erratic gauge behavior is due to lose or dirty connections. Well the sending units are new as is the wiring (maybe about 500 miles on everything at this point) so I dont believe dirt has anything to do with it. At first I thought it was the connector that plugged onto the prongs on the circuit board. I saw that the wires werent completely secured in that connector so I played with it a bit to make sure those connections were good and added dielectric grease. This did not change the gauge bounce situation. The connections at the bulk head connector did not move when I tried to push them in further.

I then ran a jumper wire from that wire that connected to the dash straight to the negative battery terminal and it changed nothing. I added a jumper wire from the ground hardness I made to the negative terminal on the battery, still no change. I added a jumper wire from the cluster housing itself to the negative battery terminal and still no change.

However, when I ground the sending unit wires to the body via a jumper cable the gauges go to full peg.

Any ideas on what this issue is?

I have spare gauges that I could wire up to see if it is indeed a gauge issue...

The only idea I have came up with for this issue is this:
The Fuel gauge is grounded through the sending unit, to the fuel line to the body to the battery. Where as the oil and temp gauges are grounded through the engine to the battery. Could it be possible that the negative battery cable has just enough resistance that it would give the erratic readings at the lower voltage of gauge reading? The negative battery cable is an original one, that was the only cable that was not replaced. I could test this theory by adding an additional wire from the engine to the battery. Does this sound too far off?

is there anything else I am missing?