I am looking at a '90 with an automatic and the sport button. The first question is should the light (green S) on the cluster go on and off with the button. It does not. It seems to be on more than off. The current owner says that there are intermittent trans troubles including lack of reverse. You can hear something spinning in the trans but no movement. But yet he states the other day, reverse was fine. The other problem is that on some gears, it seems to not have power and other times it does. Could it be electronic since it is intermittent or is the whole trans toast? Any help appreciated since I am not familiar with these at all. Thanks.

I wouldn't even consider the car. I've seen several of that vintage that have issues with the "S" light being on, flickering and somewhere in between.

Most will only start in second, due to the problems the TCM is seeing.

It could be electronic, or mechanical. Reverse is strictly mechanical, as there's no electronic control over reverse in that year. The "S" light is on due to electronic.

You could need a trans, TCM or any number of wiring/sensor/solenoid issues, without proper diagnosis it's hard to say. But it does sound expensive.