Need help with steppers on used machine

I just acquired a used Techno router, and the steppers seem to run rough and lose steps even at low speeds. The machine is old, but the steppers and electronics were replaced in the past year by the previous owner. At first I thought the machine itself was causing the problems, but I removed one motor and the axis slides easily without the motor on, and the motor still runs intermittently at all speeds (even around 60 or so RPMs). I'm running Mach3, and played around with as many settings as I could find, but to no avail. The previous owner who did the work builds and sells well-known CNC kits, so I'm assuming he knows what he is doing.

The machine has 2 mm pitch ballscrews. Am I too optimistic to hope to get reasonable speeds (at least 50 ipm) out of this machine using steppers? Would I need to upgrade to better steppers/electronics to do so?

Thanks very much for any suggestions yo might have.