Toyota Repair: 93 4Runner 4x4 auto transmission problem, hydraulic problem, shift points

Question
My 93 4-Runner (auto, 4x4, 129K miles) has started dropping out of the highest gear intermittently (once or twice a week) on the freeway.  I'll be driving along at about 70 mph and suddenly I feel the car downshift and the rpms jump from 2500 to 4000 or so.  Sometimes, if I slow down and drive for a few minutes, it will shift back up into the right gear; sometimes I have to get off the freeway and turn off the engine.  This happens whether the engine is cold or warm.  Also, the car sometimes won't even shift into the highest gear for a while, though more often when the engine is cool (a relative term, as I live in So. Cal).

I took it to the transmission shop, and they dropped the pan and hooked up the computer but found nothing wrong.  My husband just replaced the heads and had the valves done, and I hate to saddle him with another major job.  Any ideas?  Thanks!

Answer
I recommend that you have a toyota dealer try to diagnose the problem, it sounds to me that there may be a serious transmission problem, this is an ECT(electronic controlled transmission) the shift points are determined by the computer, if there is a control problem the computer will set a trouble code and either the check engine light will stay on or the O/D light will flash or stay on, if this is not the case then there is some sort of hydraulic problem a possible internal leak.
Unless there was a complete lack of maintenance of the transmission, this kind of problem should not be happening at this mileage so I'm kind of stumped as to what the problem may actually be.