I've gone through exactly the same process as you Bruce. I have the MDS 542 drives, 3nm motors and PSU's from Zapp

I bought a laptop to use with Mach3, had nightmare issues with missed steps and stalling even while the motors were on the bench at very low speeds. I got in touch with Pminmo who was extremely helpful and got me at least working. Running on the bench was much better, in fact I was pushed to make the motors stall. Putting them on my machine however, I had to run at very low speeds to get smooth motion. I could jog the X maybe 150mm before it would miss or stall, jog again and it would be fine for maybe 500mm before missing or stalling, jog again and it might instantly stall, no set pattern.

In desparation I ordered a SmoothStepper from the states and what a transformation. Before, the best I could get from my Ballscrew X axis was 1000mm/min, now I'm up at 3000mm/Min and it could go faster comfortably but it seems fast enough for me to leave it set at. My rack and pinion Y axis, whose motor turns slowly, would stutter randomly, that is all gone now, smooth in all three axes. Just running Mach3 with the SS attached and no machine, the toolpath display seems smoother running.

I know this doesn't help you solve your problem but this little board has transformed my laptop controlled system and I'd bet it would do the same for you if you cannot cure your problem. At the end of the day Mach3 and Laptops have an uneasy relationship and even if you get yours optimised to the Nth degree, I feel it will always be marginal. Cost me £130 total with all taxes but worth every penny. I know I could buy a S/H desktop for the same money and probably have zero troubles but I don't have the option of using one, I simply HAD to use a laptop because a desktop could not survive for long where it would be living.

Worth thinking about.