Tom, Carl is correct; the motors, when wired bipolar parallel can handle 5A (SQRT(2) X 3.5 = 4.9A). A 72 VDC power supply is okay (32 x SQRT(8.5 mH) = 92, but the G203v is limited to 80VDC). Verify that you have at least 22,000 uF of capacitance on the power supply to filter out the AC ripple (80,000 X 5A / 72VDC = 5555 uF per motor). You'll need to use 120K 1/4W current limiting resistors on terminals 11 and 12 on the G203v. (117K is ideal, but 100K to 120K will work just fine.)

The torque chart indicates that the motor should be able to run at 500 RPM with excellent torque.

Does the ball screw have a single thread, or are there multiple starts? The easy way to figure that out is to mark the ball screw and then measure how far the ball nut travels per revolution of the ball screw.

The G203v requires 2,000 pulses per revolution. If the ball screw actually moves .333 inches per revolution, each pulse would move the ball nut 0.0001665" (0.333 / 2000 = 0.0001665"). If the ball screw moves 1" per revolution, each pulse would move the ball nut 0.0005".

By comparison, the Shopbot PRS-Alpha with a 7.2:1 gearbox moves an axis about 0.000327" per pulse.

Round stepper motors are much less desirable than square stepper motors, but I would not like to recommend that you spend $850 to buy four new Oriental Motor PK299-F4.5 motors unless you can verify that there is no binding and that all parts are well lubricated.

Verify that the acceleration is slow in Mach 3. Set the rate to allow 1 second for the ramp time. You can always increase the acceleration after you verify that the axis will move fast enough to do the job.

You have two motors on the X-axis, so if the gantry weighs 200 lbs, each motor is responsible for 100 lbs. Using Mariss's formula on the Geckodrive web site, that size motor should be adequate for jogging speeds up to about 500" per minute.

I'm guessing that something is either binding, Mach3 is not properly configured, or that you're moving much more mass than you think.

A friend in Boise, Idaho, built a machine that had a much heavier gantry that ran just fine with PK299-F4.5 stepper motors. He replaced those motors with the PK2913-F4 model. (He also used the G203v stepper drivers and a 65VDC power supply.) His machine has ball screws and linear guides.