722.315 transmission shifting issue.

[dieselsmoke300d]

Hello everyone. Been a while since I've used this forum. But I have a 1985 300sd with a 722.315 trans in it that's had an issue since I bought it last year. And it's been sitting. But its only got 101,000 on it and I'm not sure why its flaking out on me. I'll be driving it and it'll go from 1st to 2nd no problem, then when it should be shifting into 3rd it just kicks out and slips like a hippo on ice. If I floor it, it does try to downshift. I've repaired all the relevant parts of the vacuum system and played with the modulator pressure properly and improperly. Oil levels good. Shift linkage is good. Bowden cable's good as far as I can tell. No change in behavior.

Any ideas?