Sorry for assuming you used Mach3, from my limited knowledge the settings appear to be okay, you've probably been at this longer than me anyway. I would be into trying a different motor and swapping the M542 just to eliminate those from the equation. If you think it's maybe a screening problem why not run a temporary cable between the motor and M542 that is well away from everything else. There was another thread a few weeks ago about rubber mountings and I suggested using heatproof silicone sheet like this SILICONE RUBBER SHEET 200MMSQ, 1,1.5,2,3,4,5,6,8,AND 10MMTHK | eBay, but the opinion was to bolt the motors straight onto metal.