I usually write the code to move pretty fast (20 mm/sec) and slow it down at the start. Actually I had another go at it, and this time I didn't even change the feed rate. Similar result, after an hour or so of milling using a fine point, it missed some steps or something. Anyway it got missreferenced by about 7 mm in the x axis. I was running at 80% of 20 mm/sec. I will check my driver etc., but it was working fine a few weeks ago. The way it happened, i suspect that the error occured when the tool was picked up and moved to another area of the toolpath. Doesn't it move quicker then?
Aaaargh...