Random Z-Axis plunge

[Jonathan]

I'm sure I posted in this thread after the original post...must have not sent :(

If the error is occurring it the same point in the G-code, which would appear to be the case, then it's likely to be a tuning problem. The error would be constant since it's the same command causing it each time. Assuming the Z-axis stalls at the same point then the error will be repeatable. Try just running the few lines of G-code round where it stalls and see if it happens again.

I very much doubt it would be backlash since errors due to backlash are not cumulative and 10mm of backlash would be rather silly!

A couple of weeks ago I had a similar problem in that my machine wouldn't get past a certain line of code in a spot drilling pass. There didn't seem to be anything special about it, just a rapid move in X and Y. However after noticing that both X and Y would stall every time the machine got to that line I tried just running the same line and line before on it's own - sure enough it still stalled. Naturally checked all the wiring and the usual stuff, which made no difference. Reduced the rapid feeds on X and Y which worked only if I went to about 60% of the normal speed which is a bit pathetic. Tried running the code zeroed at different points and it still stalled so couldn't have been a mechanical fault.
In the end I decided to switch to LinuxCNC and a couple of hours later the machine was running just fine at full speed.