transmission vacuum leak?

[66TBirdNC]

I recently purchased a 66 with COM ( not C6). Transmission was supposedly rebuilt 10 years ago and not driven much since. i had fluid changed and pan screen cleaned right away. Type F fluid. When warm all is fine, but when first driving for the day ( 50 degrees, not not real cold) transmission resists shifting to 3rd until about 45 MPH. Once it has shifted a few times, shifts at about 38 MPH. Also, when 'cold' like this, will downshift to 2nd while driving along. Once the engine temp is up--maybe after 3 miles--shifts up and down fine....and no more downshifting

Is that enough to tell if it is a vacuum problem, modulator, or worse??

Comments??

thanks