The '89 won't have a "real" oil pressure gauge, on the engine is an oil pressure switch, it is either on or off.
The dash board gauge will either show low(switch off) or about 1/2(switch on).
The pressure that turns on the switch is about 5psi.
You could have a failing switch, or your pressure is dropping below 5psi briefly.
If oil pressure gets too low the valve train will make more noise, if you don't hear that then probably a switch issue.
If the gauge needle is jumping above 1/2 way then probably a gauge issue.
Some think they are seeing real oil pressure because needle can go up slightly with RPM, what they are seeing is the voltage increase from the alternator, it is an electric gauge, and FYI, that can be a sign alternator is going out, voltage shouldn't change that much, that's the point of the voltage regulator.
Also check to make sure the ground strap still has a good connection from head to firewall, that's the main ground for the gauges.
If it is missing or loose the throttle cable can be a ground, i.e. tapping throttle steadies the gauge
It's a long shot, lol.