Gary is right in that there is a difference between ball screws and ACME lead screw and the difference is huge under load.
Mr.Chips wrote: Y axis (two ball screws, nema 23’s) to 300 IPM
"Mr.Chips"]X axis ( two start acme rod, nema 23) to 187 IOM[/quote]
I assume the Y axis is pushing both the x axis carriage which also carries the Z axis assembly and does so with less effort than X axis drive can push the Z axis assembly around. Also, even though a 2 start thread is twice as fast as a single start thread, it takes twice the power (roughly) to turn the thread at any given rpm and acceleration loads are even higher due to the inherent friction of an ACME nut vs. a ball nut.
If it is possible you might try gearing the X axis down if the drive assembly is belt/gear driven. I suspect your drive motor is capable of far more rpm than you are getting out of it and giving the motor a little more mechanical advantage might get you over the hump so to speak.
One thing you might check is to make sure your X axis stepper drive output amperage is set correctly for the motor it is driving. If the NEMA motor is rated at 3.5 amps and the driver is set to a lower output you will experience the kinds of problems you are having as well.