I have recently been experiencing what seems to be really erratic gauge readings from my '88 N/A. My oil pressure will sometimes spike really high, like above 110 psi, then it will drop down to lower levels. The coolant temperature will suddenly drop really low even though I know the engine is warm and at normal temperature. These 2 things seem to come and go, but happen during every drive or even short run. I have also noticed that my voltage reading on the gauge is only reading about 12.5Vd, It used to run about 14V. I tested the battery while the car was running with my multimeter and I get a 14.3V reading, but the gauge in the car still reads about 12.5V.

Does anyone have an explanation for what I am seeing?

 

There is a set of grounds on the driver shock tower sort of hidden by the trailing coil. Clean them up.

Also check the board at the back of the gauge cluster. It is possible that one of the round plugs has become loose.