Wish you the best of luck determining the problem.

I chased mine for months. I had two pressure gauges hooked up to the pressure ports on the case and monitored the line pressure / modulator pressure while I drove.
I also had a tee in the vacuum line to the modulator with a line run through the firewall to the console. I had a bleed valve and vacuum gauge hooked to this line. I could adjust the vacuum, the modulator saw, and monitor it by bleeding off vacuum. As I bleed off vacuum the modulated line pressure would rise. This would overcome the restriction and let the trans shift. Of course I did not know at the time why the trans would go ahead and shift other than I knew I was raising the pressure. I think this is probably when I also realized that manually shifting it also raised the pressure. But it finally got to where it would no longer shift. I was in process of buying a rebuild kit when I found the forum that suggested to look for the restricted screens.

With all of the above being stated, have you considered the vacuum modulator? If it is not working / adjusted properly or if it is not seeing the proper regulated vacuum signal, the trans will not shift properly. Also there is a "vacuum valve box / regulator" hooked to the throttle cable down near the injector pump that regulates the vacuum to the modulator (much like my in car adjustable bleed valve did). If this regulator is not bleeding away vacuum from the modulator, the trans does not know to shift.

And as others have stated, if the bowden cable is not working properly, then you will have shifting issues as well.

Alot to take in, here in this reply. Hope it helps.