Low oil pressure

Hope someone can help me..i have a 1991 300 E 4 matic with 207,000 miles on it-the car has been lovingly-and expensively-maintained...I recently took it on a 1500 mile road trip-on the way back, my oil pressure gauge was acting up-at low revs-below 2300 or so-I would have close to no oil pressure-at higher revolutions I would have the normal 3 reading on the gauge (all of this occurs AFTER the car is warm) . With judicious downshifting, and driving home after midnight, I managed to keep the oil pressure at 3 the whole way home-and there he sits....I have an appointment for Oct 3rd at the local dealership...however, if i had a better idea of WHAT the problem is, I might be able to get it fixed sooner by going to an after-market mechanic ( I trust the dealership to accurately diagnosis) I am not certain that i trust this after market place to diagnosis correctly (afraid they may attempt to find non-existent problems-although they DO have a good reputation)....So far
my suspicions are 1) oil pump problem-or 2) -main bearing...
There is a test I think that can determine this-a trickle-down test? How would that work?
Thanks for your help
Laura
p.s. I swear I am gonna HIT the next person who tells me to "get rid of that car"-which so far is everyone i know.....