OK, so in terms of wiring a motor bipolar-series versus -parallel, what does 'fast' mean?

My problem is that on occasion my motors lose steps when motoring round a corner (X to Y travel transition) - motors drive a compound table.

Setup:
Motors: SY60STH88-3008BF
Driver: PM542 @ 1,600 pulses per rev, 2.37A(Peak) [1.69A(RMS)]
Supply: 43V
MACH3 motor config': 627 steps per mm, 500mm per min

So:
If the motor data sheet gives bipolar-series 2.1A and bipolar-parallel 4.2A are those currents peak or RMS?
Given above settings, should I wire the motors Bipolar-series or -parallel (and why?) to, presumably, get the maximum torque within my desired 0-500mm/min operating range?

Thanks in advance.