With your car being a 1983 model, you probably don't have the same diaphram that I have on the turbo, as mine is a cali model and it is for emissions.
The vacuum leak could come from anywhere, so I would start by working backwards, first starting at the modulator. I would buy a Mity Vac (or equivelant) and pull vacuum up to 20psi at the line going to the modulator. Then I would let it sit for a minute to see if it holds. Then, I would work this way backwards until I came to a "T" of some sort, and disconnect and plug anything not related to the tranny modulator vacuum while still ensuring the source of the vacuum to the modulator is still connected, then drive it and see what happens. Thsi is what I did and tranced it to that faulty diaphram on my turbo. I would bet that your problem is vacuum related as it seems the vast majority of tranny issues are.
I hope this helps, Adam Bush