<VV> Powerglide upshift flare up

[shortle]

On my 1963 Rampside ,1965 95HP 3.55 rear axle Powerglide, on light throttle trans will flare up a bit before shifting into high. Problem is most noticeable after warmup. On heavy throttle (when upshift occurs at about 40 mph) trans shifts very well. I rebuilt this trans about 6,000 miles ago and only in the last 1,000 miles this problem has popped up. Trans fluid is very clean and level is proper. I have the FC trans cooler and I am using the aluminum finned pan. Any suggestions on what to do? It doesn't seem to be causing any other issues but I am getting ready to drive it to the toss in So Cal and would like this remedied first. As always, thanks for any suggestions or recomendations.
Timothy Shortle in Durango Colorado