Skipping movements intermettently, & work offsets

I had another play with the engraver today, and managed to produce enough good parts to keep the customer happy. I also ran the B programme repeatedly with no cutter, and the B in the graphics panel zoomed up to nearly full screen. Unlike yesterday the system now recognises the M3 command - but I changed absolutely nothing! First run, no problem. Second run the B went wrong in exactly the same way as before, and the wrong path was shown on the screen. So it must be a programme fault which is occurring in the sequencing of moves before the moves are issued for action. I imagine there is some kind of look ahead which is losing track of where it is in the list. On the third run the last G0 back to 0,0,0 got missed. This was very obvious even without taking a cut, and again showed on screen. When I homed the machine it hit the end stops again.

I ran the programme another twenty or so times to see if I could note more information from all the coordinates on the Diagnostics screen, or get a screen grab of the wrong path, but it refused to go wrong again. I then did about fifteen production runs with my long rotary programme with no errors (which gets the current production done plus spares for the future).

So I'm quite sure the error is happening in the programme, not the drive electronics, but I can't reproduce the fault reliably.

I'm unlikely to return to this for some days now as I have other projects in need of attention, but if anyone can suggest further experiments to help with diagnosis I will try to run them.