screening / glitchy operation - sanity check

[dsc]

I'm using a HBS86 + 8Nm motor with a 68VDC supply and driving this with a PIC based uC system via a PWM output. I've coded up a ramp start function, which generates a PWM with a variable frequency to ramp the motor up from 60RPM to 240RPM. On the scope all looks good, on the motor it works, but I can hear the motor sometimes glitching, as if it's loosing steps. I've even seen a 'positioning error' once on the driver, it needed a power cycle afterwards to get things going again. I'm using screened cables, but currently haven't got the screens connected, so I thought maybe this is the reason. Here's how it's connected:



I've numbered the screens, should all 1-3 be connected to 4 - E(arth) / chassis? I'm guessing I need to put a link between the E(arth) terminal and the chassis as well to get the whole chassis earthed?

Thanks in advance for any suggestions.

Regards,
dsc.