Same/very similar problem here. When jogging an X-Y table with MACH3 at full speed (400mm/min) in either the Y+/Y- direction and then, at the same time, attempting to jog either X+/X- at full speed [and completely vice versa] there are a lot of loud horrible mechanical noises and the table clearly stops moving correctly. This is 100% repeatable. Not seen this happen when machining a piece - that said I'm not aware of a situation where the machine is asked to move full speed in, say, the X direction whilst it's already moving full speed in the Y direction. I assumed - for no good reason - that this was a MACH3 jogging thing. So now I'm very interested in the answer.

Setup:
Motors: SY60STH88-3008BF (x3)
Driver: PM542 @ 1,600 pulses per rev, 2.37A(Peak) [1.69A(RMS)]
Supply: 43V
MACH3 motor config': 627 steps per mm, 400mm per min