I'm out of ideas...

I've got a 3-axis Anilam 1100 setup on my Lagun mill. Everything works great except:

In the middle of running a program (either a built-in routine or a program loaded from disk), the x-axis will shift, sometimes +, sometimes -, half the tool diameter. No errors are reported and the program continues (at the new table position) until finished. Doesn't matter if toolcomp is on or off. When I first notice the cutter 'heading off', I stop and rapid to X-0, Y-0, and the cutter is always off of part x-0 (one way or the other) by half the tool dia.

It happens most often when taking a heaver cut. I can machine plastic OK, but switch to steel and the problem almost always surfaces. Have tried to load the servo by the handwheel on a practice run, but could not duplicate, so maybe it's vibration related?

Have cleaned and reseated any connecter or card related to the x-axis, and checked to make sure the encoder is not loose.

Has anyone run across this before?