First try inspecting, cleaning, and cycling the connector on the oil pressure sending unit. If that doesn't work, replace the sending unit.

As cars age, connector problems - dirt, corrosion, cumulative stress, etc, can cause intermittent electrical faults, particularly on non-balance shaft inline fours, because the second order unbalanced vertical shaking force beats the hell out of everything attached to the engine and can actually cause connectors to wear!

My Cosworth Vega has consumed two oil pressure sending units, the spades on two voltage regulators and two sets of voltage regulator terminals on the engine harness eventhough the alternaltor is isolation mounted in rubber bushings.

Duke