I've got a "flakey" jittery oil pressure gauge also (Ford Ranger XLT 2002, 3L, 6cyl).
Exact same symptoms ... only flakey at idle, after driving for a while on freeway.
Turn engine off, and then back on it goes away until next idle.
If you aren't looking at the gauge, you'd never know there was a "problem".

It's been doing this for about 4 or 5 years now, so I'm kind of think its not
a big problem, but seeing that gauge get jittery still makes me kind of nervous,
and I wish it could be fix, but I don't feel like spending a bunch of money on it.