<VV> PG Question

[shortle]

Hi All,
   My 1963 Rampside with 1965 95HP, 3.55,PG recently started "flareing" while on a light upshift from first to second.On heavier acceleration it seems to upshift quite well. I noticed a couple of cars in Ventura this last summer doing the same thing when they were driving by. Can anyone provide any insight, suggestions, or known fixes. This is something that just cropped up out of the blue. I rebuilt my PG about 6,000 miles ago using Harry's film as well as shop manuals for guidance and up until recently it was doing quite well. I am trying to get my vehicle ready to drive to the Fan Belt Toss (about 825 miles each way) and am hoping for a trouble-free drive. Anyone?
Timothy Shortle in Durango Colorado