I kind of have the same problem. I've only started doing 3d stuff and noticed the machine is kind of jerky when doing the carving. I figured it's the programming because the surface does not show signs of hesitation marks...hmmm hesitation marks, I think I watch too much CSI...anywho...and comes out fairly smooth, I say fairly because i know I could program much cleaner cuts but don't want to. I noticed today when cutting an angled neck pocket that it was jerky but also realized that my z axis was tuned to 40 ipm and the Y was 60, so I figure it's a timing issue. It looks jerky because one axis is stalling waiting for the other axis to catch up. But I'm no expert so I could be totally wrong.