Well as of lately I've noticed a problem with my transmission having issues with the 2-3 shift.

A bit of background info:
The transmission is about 1.5 years old, full rebuild:band,cluches,shift kit, servo, beast sunshell, larger stator shaft, sprag, bushings, bearings, etc. Basically everything was brand new on it except the wiring harness, housing and input/output shafts.

Anyways, as of lately I've been having issue with it wanting to stick in 2nd. It shifts fine at anything 5k and below but on shifts between 5000-6000 it will hold that gear when I let of the throttle slightly.

It's not slipping, I know what that feels like, but rather it feels like the PCM isn't commanding a 2-3 shift fast enough.

Some other info
-The fluid looks fine and is bright red
-I have no problems with other gears, and lock up is fine
-I have an ss3600 Yank converter
-My transmission is vacuum modulated

I'm worried it's something with the Vac Mod. I know you can adjust shift firmness but I never have since my builder set it up for me. It almost feels like there is no line pressure or less than there should be.

I'm leaving a lot of info out but bottom line question is:

Is there something in the tune or with the vacuum mod that would hold back a 5-6k rpm 2-3 shift?