Wow Jazz.. Lots to chew on...

Ought eliminate a couple of potential probs tho... I am not daisy chaining the motors/drivers with power, all have there own separate feeds/wires.. Not sure what the voltage limit is for these motors and I have been frowning a smidge at the 24v power supply as a possible weak point.. But the y axis is not stalling, and my last cnc was driven by just that one 24v psu, and it drove 4 x 3Nm motors then with no probs like this one...

Was running Mach3 at 25Khz, ramped it up to 45Khz in trying to fix this prob and it made no difference (using a dual P4 board, 2 gig ram with XP in desktop PC mode) so I left it at 45KHz..

Driver test is fine..

By ground, you mean earth right? Connect 0v and or ground to earth? Read this many times but when I look into my box of tricks, there are an awful lotta grounds and 0v points, more if you eyeball the PSU's too.. Pass the aspirin...

Mid band resonance... Is this a resonant frequency of the machines mechanical structure? If so I doubt this is a problem as the motors are very smooth and pretty quiet (apart from that dull thump caused by backlash comp in Mach3) until the very sudden growl from the nema 34's when they stall.. The fact that they both stall, and bearing in mind they are slaved together in Mach3, still suggests something electronic tho... Also as both the y and x are running at full speed simultaneously suggests it might be power, so will get a new battery for my multimeter first thing tomoz...

Will drop the steps as you suggest and check some voltages in the a.m. Starting to get intrigued by earthing, really feeling the need to get that one outa the way..

Steve..