New Reman trans with TQ (dynoned) , new flex discs, CSB and chain

I can't duplicate it with consistency but sofar picked up on this 3-4X. On the last time, I've observed this, when at freeway speeds and I coast or just feather the throttle, it seems to cause this vibration/shudder, I can pickup. Once I apply some throttle to it, it smoothes out.

Easy answer anyone ?