Auto tranny keeps getting stuck in gears

Sounds like your transmission valve bodies need a good cleaning.
I have never removed the valve body on the z31 auto trans, but I know other Nissan vehicles, the valve body can be removed from just simple dropping the pan.

But I suppose before you put all that into consideration, double check your TPS (throttle position sensor). Refer to the Haynes or the FSM for the procedure.

Good luck!

Also check the vacuum line at the back of the engine bay, there's a hard line coming up from the tranny and then a rubber vac line connecting to that. If you are standing on the passenger side looking at the tranny dipstick it'll be up around, like, 10-11 o'clock I think. If this isn't hooked up you can have all sorts of shifting weirdness.

Also, in case people are searching for auto tranny threads, this can't be said enough. If you buy a used car with like 80+K on it, the auto tranny is working fine, and you have no idea when or if any transmission maintenance was ever done on the car, don't change the fluid. Just leave it. Don't touch it unless you overheat the car real bad at some point. At this point, changing the fluid will just free up a bunch of sludge which will choke your tranny.

edit: while we don't advertise it too much there are a number of us here who rather like our auto Z31s (especially those of us who DD them)

This is true. I am thoroughly enjoying my JDM 2+2 Turbo… which is an automatic