Run the mach3 driver test, do you get smooth results?
I'd say it's one of three things.
2)Something in the PC interfering with Mach3.
3) still might be an acceleration issue. Drop it in half again and you can eliminate this one if it does the same thing.
Since it's only happening on the Z axis, you could also try swapping the X and Z wires on your drives and see if the problem moves to the other axis.
What drives, motors, and what size power supply are you using?
And if you leave the task manager open while cutting, does the CPU usage ever hit 100%?
Also, Art always recommended a 2Ghz or faster CPU for any kernel speeds over 25Khz.