In another lifetime I use to install and service DRO's and CNC controls (Bandit, Sony for you old timers) and I believe what you have there are glass scales given the size. Every once in a while you need to take the cover off and wipe down those scales with some denatured alcohol. The scales are protected, but not sealed, so grit can and does get up in there. That said, the first thing you want to do, as another poster attested, is to swap the cables on the back of your readout. If the problem stays with the same axis, its the readout itself that's giving you the problem, and conversely if the problem changes axis, then the problem is with the scale. Since the trouble that your having is intermittent, I'm first inclined to suspect a poor connection somewhere. These can be difficult to troubleshoot as the poor connection may be something like a solder joint on a circuit board. You may want to just wiggle the cables a bit near the connectors themselves to see if you can force the problem to occur, but without a schematic it would be very difficult to troubleshoot any further unless you have experience with these kinds of things. If you have any other questions about proceeding, give me a shout.