Well I think you have been very lucky to get it working anything like stable if you haven't got sheilded wires and correct form of grounding.

It's not difficult really and it's more attention to detail than anything with a few correct procedures.

Earthing is all done thru the Mains earth but what you don't want is individual earth wires going to several earth points on the machine. You need ONE point which connects directly to the MAINS earth. This then acts like a STAR Earth point for all other earths Ie: shield wires to connect too. This then avoids any earth loops which flip drives etc brains out.!

ALL Signal wires and Motor wires should use Shielded cable which the shield wire then goes to the STAR point.

It's that simple really but makes a big difference regards noise handling.

One more thing to note is that you shouldn't confuse DC Negative with AC Earth. All DC Negatives should be taken to the DC supply Negative and not to mains EARTH.
Also the PC or it's case should NOT be Earthed to the STAR point. The PC will have take it's Earth from the mains directly so treat it as separate and best put on another separate Plug.