In post #5 you say you adjusted the VCV and got better shifts. Did you mean the trans modulator instead? The vac and bleed off thereof caused by the VCV does affect your upshifts. But it bleeds down to zero when you hit the throttle so the upshift is smooth. What reading did you get if you adjusted the VCV before and after adjustment - and is it still bleeding off when you hit the throttle? The pump should be putting out about 20 iHG and you should be getting between 10-15 iHG after the VCV.

Rereading the posts, if you are having trouble upshifting with no vac - if it is slipping rather than flaring, it sounds like a transmission problem. Next if you are getting better upshifts with new, thicker transmission fluid - and then when the new fluid warms up and gets thinner, the shifts begin to slip - it again sounds like a transmission problem.

I believe you can rectify a slipping a little and for a short time by adjusting the trans modulator for crisper shifts. Would try this now.

But I am guessing you have bad clutches, etc from wear and that is the real reason the tranny is slipping.

However some forum member who knows the insides of these trannies should opine; about that I know not much.