Bonkers Oil Pressure Gauge- W124

[chazola]

The oil pressure gauge on my W124 has started acting up when the engine is warm (after 1/2hr- 1 hour or so). At idle, in drive or in R, 3 or 2, the pressure bounces around 1. Doesn't drop below 1 though. B4 today it was always around 2. If i shift into N or P it goes back up to 2. the RPM also drops to around 500 when the pressure is at 1, then back up to around 7-800 in N or P.

At normal driving the meter is pegged back on 3.

oddly, it started this just after i'd just filled up with gas. I was getting the surging idle a couple of days ago, again when the engine was warm, which seem to stop today.

Last oil change was @ 2000 miles ago, at dealership.
I have pulled the R16 resistor trick yesterday, gonna put it back it when it stops raining.

91 300E 192k with leaky front cover.

any thoughts?