transmission slipping/not shifting

Correct the fluid level first and foremost on any automatic transmission before conducting diagnostics. If not, you will be chasing your tail forever on this.

a friend also recommended stop slip additive? it seems to have mixed reviews from the research ive done and only for a short term fix before making everything worse. would it be best to just leave it out, or give it a go

It might be good enough to help you make it to the transmission shop. There really aren't many solutions when these auto transmissions start to fail.

+1. The good news is that a rebuilt one will probably outlast anything else on the car. The bad news is that unless the body is absolutely rust-free and you really want an automatic z31 (when some idiot hasn't overheated it the auto is actually totally solid and a perfectly good tranny for a DD, especially if you have to drive in traffic a lot - I decided to keep mine in my 84) you'd be nuts to put that kind of money into that car.

Think pretty hard about what you want to do here… the 5-spd swap is not really a good "my first big car job" if we're talking about the car you need to get to work and stuff. You might be better off getting out of this car before you get further in money-wise…

Guys stop being so apocalyptic on the guy haha

He clearly said the problem comes and goes, that leads me to think it's not a friction issue, but rather a line pressure issue leading to improper clutching/lockup.. lucas and amsoil both make additives that improve the friction and anti-aeration qualities of the fluid, considering the age, this won't hurt to try

But my suggestion as before, is keep trying to drive it, and see if it gets better, it takes time for new fluid to displace all the crap from the valvebody

NO, its the end of the freakin world already.

How many miles? Any fluid leaks?

replaced the modulator yesterday. problem is going away more and more. but it seems to be now that after the fluid change, my speedometer is cutting in and out and causing the transmission to shift when it isnt needed. think the speed sensor might be going? there are some sketching looking wires underneath that i have to tidy up a bit, and hopefully solve that issue. but the transmission is working properly for the most part (minus the first 30 seconds of movement where it doesnt want to shift). might be the speed sensor reading the speed incorrectly and making the car think its going a different speed and not going to shift. because when it does work, the shifts are smooth and stay in place well.

and btw, this car is solid. 158k miles, no leaks, garage stored for a long time, minimal budy rust (minor spot on driver side rear quarter panel and around the antenna) but almost perfect body wise besides that, minor spots of rust underneath but nothing structural, and then minor issues like bushings, cracked tail light, power window assembly, etc need work. i wouldnt have bought the car if i didnt feel like it would be a good car for me to have for a long time and to care for/build up over time. yes its my first car, and i may end up putting "too much money" into it, but i want to have a car i can get my hands dirty with, learn the mechanics of everything, build up, and feel proud of when its finished years from now. all these "you are going to put too much money into it" comments just get skipped over. i just need to make this transmission last long enough until i can afford a 5 speed swap. if i end up putting tons of money into the car, oh well. i love it. it has its quirks. and the quirks are why i love it. theres something that draws me into the car and makes me want to drive it everywhere. no other car ive been behind the wheel of do that like this car. so if it takes some money to get it running perfect, so be it.

EDIT: found a wiring nightmare underneath. melted wires, wire nuts, etc. wire from the speed sensor was melted, wires coming from the overdrive controller were spliced and wire nutted, and so on. going to be cleaning up all the wiring i can and see how that works. already fixed the wires coming from the speed sensor, and the speedometer has been working fine and the transmission seems to be working better as well. takes one good hard push when first starting off to get the shifting to start, but once it does it shifts smooth and acts like it should when the car is moving. still has some slip when accelerating from a stop (alot like a newbie on clutch that waits too long to release it), and some hard shuttering from the transmission wanting to grab and worn out suspension bushings allowing everything to move around like crazy. but its enough to get me where i need to go until i can afford to replace it with a 5 speed. will be replacing the fluid again within 3k miles to get as much crap out as possible (drained some out when i replaced the modulator to have it at the proper level since i didnt do a fluid flush, just a drain so it was actually overfilled a little bit and it was a pretty dark red color) and might add some stop slip or tranx additive before that fluid change if i can find one that has had consistent positive results.