Well, I think I got it figured out. I tried a ton of different settings but the one that seemed to fix the problem is the shuttle speed from .005 to .05. With that setting changed I can run up to 50 IPM with no problems. I didn't test any higher. After making that change I kept changing velocity, accel, backlash and such but the fix held unless I changed shuttle back to .005. So, I think that's the one! I cut a nice, round circular pocket with BLC on at 24 IPM and it came out round within a few thousandths. Good enough for me!
Thanks very much for your help and pointers. I wouldn't have thought to look there without it.
Thanks,
Jason