Are you getting any drive errors? Has the brake set? It sounds like the unit doesn't know the correct position. When the fault happens what motion region are you in? Is this an A-series drive? What does the motion excuses screen say? Check screen 3 in motion info. The VcMD, VFB, and VENC should all read fairly close to each other. One last thing, if you have a motion adaptor board, you may want to switch it in. You do have to perform a REINIT DRIVE if you do.