[ModMech]

Beginning in the mid '80s, Ford stopped using "real" oil pressure gauges, and switched to an "idiot gauge", that either reads pressure, or it does not.

It is VERY common for the oil pressure switches to go bad, and cause erratic reading of the gauge especially at idle. The switch and wires are also very close to the oil filter, and are subject to damage during service work by careless persons.

Check for any obvious damage to the wiring near the filter/switch area, and if none if found simply replace the switch.