After rebuilding my 700r4 in my 86 Camaro, I still have the same problem in that when it goes into 4th gear, at the slightest touch of the gas pedal it changes back into 3rd, regardless of what speed you are doing.
Fitted a new TV cable and it is still doing the same. Adjusted the cable several ways as according to the manual, Transfixleo etc still to no avail..
Getting close to slashing my wrists or torching the car !!! or both !!!!
Please, somebody help!!!!!

 

If you are talking about at high speeds you are probably just running into the no-WOT feature everyone complains about on these. If it is a any speed then you are looking at an internal calibration issue which on an '86 doesn't come as a big surprise to me.