after operating some surface runs for about 3-4 hours of run time without a hitch(machine on for about 6 hours total) I ran into a glitch. at the end of a operation the z position jumped about 2.5 inches on the readout. i reset the z and run another operation for about 25 minutes. and at the end it jumped again - same direction and about the same 2.5 inches. from here i reset z three more times trying to run another operation finally shut her down as it was getting time to quit anyways. it seems to do this if i hit the upper limit switch or in a general retract move (z+). encoder? belt jumping? other ideas for me to check tomorrow?