Not had this problem on a Supra, but most automatic gearboxes have speed sensors which 'tell' the gearbox ECU to change up or down at the right speed. These can fail or the pins on the plugs can get corroded if they are situated on a section of the gear box casing that catches the crap off the road.
The speed sensors are not expensive from the US, I only say that because when a similar problem happened to my old Kia saloon, the replacement sensor was under $40 delivered from the US, but £85 from the local Kia dealer.
On US ebay for $14.95!
http://www.ebay.com/itm/AUTOMATIC-TRANSMISSION-SPEED-REVOLUTION-SENSOR-TOYOTA-LEXUS-SCION-89413-08010-/161840198354?fits=Year%3A1995%7CMake%3AToyota%7CModel%3ASupra%7CSubmodel%3ABase%7CEngine+-+Liter_Display%3A3.0L&hash=item25ae6d72d2&vxp=mtr