Further on this… may be issues…
I started having random milling errors. It would random detour… so far the way I have resolved these is to disable the spindle control. My current working theory is that spindle control is probably fine IF you are using PWM255 mode on an 8 bit RAMPS/mega256 setup but the RPM mode may be causing corruption elsewhere even thought RPM seems to be working… So for now I would hold off on the RPM fix and use PWM255 until I confirm.