With a 3 foot long cable, I would not expect any issues due to cabling (once you get it wired correctly )
Have you tried a different PC or a different com port on your PC? Not that it is terribly suspect, but it is maybe one of the easier 'try it and test it' options. I suppose getting a new serial port for the control would be a large PITA, but maybe it has an unused port somewhere in there, that you could switch to for testing. Perhaps it might be using different hardware.
Perhaps another option, if the control has the option, is to run DNC without decimals, that is, run in units of machine resolution. Why would this be any better? I have no idea, but it would be different. Maybe decoding the decimal character causes an issue sometimes.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)