Ray,
I've had an ACS for about a year now. Had the same problem with fliers going high. I have found that if I rotate my wrist up to get my heel off the grip, it settles down nicely. Only problem, in the heat of battle this past spring, I wounded a 6-7ft black bear at 13 yards by hitting him very high. He was back an hour later but would not give me a shot. I guess in all the excitement, I forgot to get the heel of my hand off the riser. I have been stump shooting with it for a couple of months now and think I have cured the problem. If it wasn't so darn fast (and accurate) I probably would have moved on.
Jon