Sounds like your encoders. I had that problem with mine a while back. Is it happening when it goes right to left or up and down or both? I first got new encoder wires and that was not the problem so they sent new encoders and have not had a problem since. Before you do all that make sure your rails are clean. My local dealer ordered the parts for me, the machine is under warranty.
I understand that they are having a problem with the belts freying too. If you ask they will send you a new belt for free.
Marge at the yahoo group is a great help too. If you have not joined here's the link.
http://groups.yahoo.com/group/TinLizzieLongarm/