I encounter 2 problems when trying to correct the grip shifting during the service motion:
1) Grip gets too tight. It does not slip, but I don't get a full racquet drop due to the tightness.
2) Grip gets too loose, and the grip shifts during the takeback.
The coach says my shadow service motion is perfectly fine --loose and relaxed.... But I get tight when I have to toss and hit the ball.
He says it might take several months of practice to fix the habit and the fact that I can do it correctly with a shadow motion means that it's mostly a mental issue.