Toyota Repair: 1990 4Runner transmission problem, cool morning, hydraulic pressure

Question
I have a automatic 90 4Runner with 45,000 original miles.  Lately it has started having problems with what seemed to be going into overdrive.  When driving down the highway it started to go in and out of OD.  I took it to the dealer and they kept it 3 days, flushed the transmission and returned it perplexed.  Yesterday when it did this I switched off the OD and the problem stopped.  Today leaving my home it was doing the same kind of jerking in and out of gears but this was at lower speeds in the city, so again i turned off the OD and that helped.  I live in Denver and we still have cool morning temps so I don't think the transmission is overheating?  Any ideas, I don't feel good about taking it back to the dealer since most all the mechanics there are under 35 and they don't know what it wrong.  Please help.

Answer
I also used to own a 1990 four runner and had the same problem and I have seen other early four runners exhibit the same symptoms and the new is not good, this is a sign of the transmission going out, it is losing internal hydraulic pressure due to leaking O-rings and seals.
45,000 miles is not much for this year model but time is also a determining factor.