Their seems to be a problem with the spindle control of RPM. When I run the calibrate routine the RPM moves from zero up to max is uniform controlled steps . After words, when I request a slow RPM say 350 the motor goes to full RPM. Above about 850RPM the control is smooth and predictable though not getting within several hundred RPM of the S*********x input. I have looked at the pulses with an “O-Scope” and both STEP and PWM INCREASE the spindle speed (more % in PWM, more Steps in Step in the lower speeds) which points to a software problem in MACH3. If I run in PID mode, the requested RPM (S*********x) is within 1% or better. BUT the speed zooms as much a 1000 rpm higher (for about 1.5 seconds) then comes down to the programmed speed. Playing with various PID settings will reduce the zoom effect, but add 5+seconds for RPM to ramp to the new value.  I find this a major problem when using a large FLY cutter as they are notoriously unbalanced and over reving the spindle cause serious vibration.
Does anyone know if this is a setup problem? If you have a fix, I'd love to hear it, this has been driving my nuts for months.