Kyle Boatright wrote:

I was out flying this evening (Beautiful evening in North Georgia. We don't
get better flying weather than today...) and noticed that my oil pressure
was "stuck" at 100 PSI. I reduced throttle and RPM, no change, I increased
throttle and RPM, again no change. I tapped the face of the gauge. No
change.

Eventually, I shut down the radios and transponder and turned off the master
switch. The OP gauge fell to zero. Back on with the master and it's 100 PSI
again. By the time I returned to my home field, the pressure gauge seemed to
be acting normal again, so I presume it is going to be an intermittant
problem - the absolute worst kind to diagnose.

Anyway, the gauge and sender are both Rochester brand. Probably 6 years old
(4.5 years flying)with 350 or so tach hours on 'em.

How do I diagnose the problem when it recurrs?

This is a gauge with a remote sender mounted on the engine, right? I
ask that because what you are describing sounds like a wetline gauge
with some air in the line and cold oil.

Maybe the sender is dying. Can you see if the voltage in the
electrical line between the gauge and sender varies with oil
pressure? Then you could eliminate one part or the other.