Toyota Repair: Transmission not engaging sometimes, issuse, hydraulic pressure

Question
I have a 95 4Runner Automatic Transmission - 4 wheel drive.
It has 275000 kilometers on it.

My problem is tha about once every second week when I go to startup the vehicle in the morning and try to shift it into drive or reverse... nothing happens. The vehicle just revs up but doen't move. Strangely enough a few minutes later turning off the key a couple of times and restarting the vehicle, it will finally be fine, like nothing happened. It will be good until a few weeks later.
I've talk to the repair shops... they always want me to bring it in when it is doing it. Nine month ago I had a transmission cooler put in because I had an issuse with my AT temp light coming on after about 150k drive. Since then my AT temp light has not come on. My Fluid is topped up and I don't think it looks burnt.

Any ideas... I'm hoping it's not my transmission going!! Are there sensors that could be the cause of something like this?

Thanks
Craig M

Answer
This probably due to an internal hydraulic pressure loss which translates into a major problem with the transmission that is starting to develop, unless there is a check engine light on the problem is not caused by one of the control solenoids but rather a mechanical problem.