ive had the issue with the tps before and my symptoms were not shifting gears intill high rmp in the range of 4000-5000 this still might be your issue but it sounds more like something with the trany there is checks and i believe on the 90 its an adjustable tps and you would just need a multimeter to check it if mine got wet thats why it went out and they are really anti water or it could be just a loose screw on it because you adjust it threw the mounting screws and if they are loose or one isnt there its going to throw your whole shift pattern out of wack google how to adjust it to get the right adjustment points it needs to be at on your meter

 

I was a little skeptical when someone suggested the problem was the Throttle Position Sensor. But, after installing a new one today, my tranny problems are cured.

I did not realize that the autotrans was also connected to the TPS. It now shifts when it should, does not jump back and forth between gears, and now kicks down perfectly to a passing gear at WOT.