found emc ragidness problem

I have one computer 300 mhz. that i have cncpro on and working fine,
tried installing the bdi on that one but would not take. So i
installed it on a 500 mhz machine and installed just fine, connected
the drives and the motors sounded like tin cans draging on the ground,
so i thought that this is what peaple were talkig about {raged steps}.
by chance i thought to try cncpro on the 500 mhz. machine and wala,
tin cans again.Why would cncpro work fine on the 300 mhz. machine and
be rough on the 500 mhz. machine???? In the end i want emc to work.
It looks that i have major interferince problems. Would an opto-
isolator board help me in this?.If I had ground loop problems why
does the 300 mhz. computer work? In both computers i am getting the 5
volts from the computers for the gecko's 201.

Thanks in advance for your great advise.
Richard