Spindle should not slow down other than if the cutting force causes it to. Are you saying that the spindle is slowing down just due to the motion?
Try running the thread with just air cutting and see if it slows down, if it does there is an issue somewhere with your setup. Are you still using 067?
Regarding Mach4, CSMIO are or will be working on a plugin, they have the initial info for helping them as Brian passed that to them a month or so ago.
Hood