Fluctuating fuel/water temp/oil temp gauges

I  have had my 1983 GTi for about 25 years and it as always had this odd thing where the oil temp readout on the MFA increases by about 4 degrees when you turn the headlights on.

Recently it has developed another issue.  After reaching running temperature, the oil temperature on the MFA fluctuates by about 6 degrees (goes from 86 to 88 to 90 to 86 to 90 etc, changes every second).

I also noticed today that the fuel gauge and water temperature gauge dropped from about 50% to 25% for no reason (normal driving) and after a while went back to 50%.

I am thinking this might be an earth issue?

Anyone have any ideas of where to trouble shoot?

Thanks for any help, most appreciated.

Replace your main earth, Battery to frame to engine.

Replace your earth strap from the right rear corner of your valve cover to the Coil bracket

Verify that you have a good earth to your cluster (brown wire) would hurt to splice in a ground from the inner firewall behind the cluster to the connector side or your cluster connector

Lastly verify that your 10v stabilizer on the back of your cluster has 13 volts (B+) on one leg to the center leg, and that the other leg is 10V.   If t doesn't then you need a new 10V stabilizer.