2001WS6Vert

I had to have my built trans. redone, it has all the good hard parts. I let a local guy do the job. Now if I accelerate as hard as I can in 4th (outside of lockup but before kickdown) I get a shudder/slip. Dave at Rock-On has been very helpful telling my builder how to set up my trans.. If it has to come out it goes to him. I popped out the 4th gear piston and replaced that seal ring + added a band shim but it made no change. Does anyone have any other ideas that I might try before determining that it must come out? Pressure is about 210 psi (modulated). It seems like if I could get the band to grab a little tighter I would be good.