is this under any throttle condition? If under light throttle, I would suspect a partial clog somewhere perhaps or a worn plate/hole/ball. Under heavy throttle, maybe a plate/ball is wearing. I know when I did the shift kit in my Firebird (TH350), you replaced the separator plate with theirs and leave out certain balls (or add them... been a while!). If I drove it normal, it shifted normal. If I was on it hard, it would bark from 1st to 2nd at 65ish and a little squeal from 2nd to 3rd at 85ish.
Somebody else will be along with the right answer soon, I'm sure. I haven't delved into auto trannies any more than that. I just noticed my TB doing the "shuffle" once or twice, but no CEL yet. I think I'll be doing a PWM eliminator kit this spring when I change the tranny fluid.