Toyota Repair: 87 Toyo 4Runner AT shift problem, shifting into overdrive, input sensors

Question
Ted,
I have an 87 4Runner with 150K miles and auto trans. The RPMs fluctuate and it shifts all over the place.  Usually starts with not shifting into overdrive (or coming out of OD while at highway speed), then after a few minutes shifts back to OD.  When I stop at a light and accelerate again it downshifts/upshifts by itself back and forth between 2nd and 3rd gear, all the way up to highway speed again...where it won't shift into OD or shifts between 3rd and OD.  Changed the TPS and it didn't help...had the AT removed and rebuilt and it still does the same.  Any suggestions on a fix...my daily driver is becoming un-driveable.  Thanks, Dan

Answer
Since the transmission was already rebuilt with no improvement we can rule out the transmission.
That leaves the computer(ECU) and it's input sensors, an erroneous input signal will in most cases cause the computer to make the O/D light come on or flash indicating that there is a problem, if there is a ecu temp sensor failure or a speed sensor failure the check engine light would come on, are either one on?
If there are no warning lights on I would suspect a problem with the ecu itself and it may be a good idea to try to locate a used one and substitute it with the existing unit.