As the title suggests, I have come across an unusual problem with my router.

Occasionally it will change its cutting path mid-run without apparent reason.

I was cutting an a double Archimedian spiral out of 4mm plywood, it ran the first spiral of 5 passes perfectly, then started on the second spiral, it ran one and a half passes on the second spiral Ok, then suddenly shot off at an angle and continued to cut the path from another position. The two paths had be drawn in Illustrator, taken into Aspire 4.5 and calculated there, then exported as a .txt file to run in Mach 3 as a complete path, not separate for each spiral. I stopped the machine and put it down to a glitch without exploring what had happened. I ran the toolpath again in the air, and there as no problem.

A couple of days ago I was machining and lozenge shaped bowl which again I had generated the outline shape in Illustrator, taken it into Aspire to model the bowl, exported it as a roughing cut and as a finishing cut.

I ran the roughing cut without problem, changed tools and started the finishing cut, after 30 seconds or so of the finishing cut, again it displaced and continued cutting the bowl, this time I let it complete and when I returned the tool to zero, the zero x had moved to the right by around 25mm and the zero y had moved down by about 6mm. As the shape was regular, I was able to save the work by running the toolpath again a couple of times, displacing the x axis to take account of the error, then cleaning up by hand after. I ran the path a further 4 or 5 times to correct the bad cut, and each time it ran perfectly without error.

The router and computer are both on a UPS so there should not be any problem caused by power fluctuations etc, but I am at a loss to understand what is happening, I have since run a lot of other toolpaths without problem.

Any help, advice etc. gratefully accepted.