I have a 2002 Ford Ranger that doesn't have a real oil pressure gauge
- it has one of those "fake" gauges that always shows either zero or
half scale.  The oil pressure has always behaved normally (i.e. it's
not zero :-) until a couple weeks ago when we took it for about a two
hour drive.

 It was fine on the freeway but when the engine was idling in the
hotel parking lot I noticed that the gauge was "twitching".  Since the
gauge can't actually show any real values, my guess is that it's just
intermittently oscillating between zero and normal, but the gauge is
too heavily damped for it make the full swing.  No warning lights, if
that matters.

 I shut it off and it sat there in the lot for about an hour, and when
I cautiously started it up about an hour later it was normal again.
Steady pressure reading.  In a couple of days of short trips driving
around town it behaved perfectly normally.

 Until, that is, we took a two hour drive to get back home, and by the
end of that trip, it was doing the exact same thing again.  But since
then it still behaves normally for trips, even as long as 30-40
minutes, around town.

 Even a 30 minute trip seems like it should be enough to get the
engine and the oil plenty hot, especially with the A/C running and the
hot weather.  I just can't think of anything that fail after two hours
that wouldn't do the same thing sooner, and it's really difficult to
troubleshoot a problem that requires two hours of driving to reproduce!

 Any guesses as to what might cause this?

Thanks,
Bob Armstrong