6L80 engaging & disengaging

[VEHSVR8]

Hey all,just had my trans rebuilt and new highstall(3400) installed last week.
now its got the last tune in it which was great but since the rebuild 1-it whines and 2-it engages and disengages going down hill or
even if I back the throttle off while cruising down the highway.
It's like it hit's neutral for a moment(40/50yds) then re engages,can't figure it out.
All suggestions greatly appreciated.

[Tre-Cool]

sounds like it needs to come apart to be checked