Not being familiar with your equipment I'll just throw out some ideas, in the past on my CNC's I've had a D/A output card develop an unstable zero, more of a problem on our carriages and tool slides, as our spindles had (have) either a 3 phase input contactor or a DC output contactor to isolate the spindle when stopped. What type of command signal do you have? (0-10V, serial, etc) The D/A card problem I referred to were all +/- 10VDC outputs. I would probably be worthwhile to scope the command inputs if they are analog to look for irregularities.
