TH350 Upshift Problem

I'm having a problem with a 73' 350 TH transmission upshifting from 2nd to Drive while under throttle, light or hard throttle. If I gain speed up to above 50 MPH and let off for several seconds it will then shift. I thought that the detent cable was not hooked up but when pulling on it at the pedal I have spring tension and return. I have vacuum at the modulator with no Fluid in the line but I was under the impression all the modulator does is control softening the shift, not when it shifts.
This is a new transmission built several years ago along with a new engine and was only run around the block a couple times a month then was in body work and paint work for 2 years before I realized it had this problem. The motor is a 355 with 10.5:1 pistons, 512/292 cam on 110 centers and feed with a 750 DP on Dual plane maifold.
I know with a cammed engine it will have low vacuum but I've never had a problem in the past on similar setups- even some more erractic. The trans was rebuilt back to original, no shift kit or manual valve body and the motor is tuned and runs fine.
Any thoughts on where to correct this and will adjusting the modulator help.
Thanks for any advice you can offer. I know this is a truck Forum but I ran across an article in a search that pointed me here so I thought I pass it along to you'all. Don't step on me too hard I 'm running this setup in an originally owned 73 Chevelle SS.