Engine rev-ing off and on when warm

I have a '96 GMC Jimmy with >200k miles. Recently, once the engine warms up and the rpms fall to idle speeds (stopped at traffic lights, in park, etc.) the engine will rev for a couple seconds then stop for a few seconds, then rev.... It seems to be related to the oil pressure. When I stop at a light, the oil pressure drops to about 20, the engine revs until the pressure rises to ~40 then stops, then the pressure drifts down again. The "service" light never comes on. I've seen several postings with a similar problem (engine reving high) related to the MAF sensor, but I don't know how that would affect the oil pressure.
This doesn't happen all the time, but once it starts, it continues until the Jimmy is shut-off and cools down completely. Any ideas???

Check your vacuum lines, or intake gaskets. If they are leaking they will cause a lean condition and then the engine adds fuel to correct the air fuel ratio causing the engine to rev up

When was the O2 sensor last changed? A faulty or wearing out O2 sensor will tell the engine wrong EG readings and the engine will rev up and down or studder trying to compensate.