Jacko;
Been there done that. I have a HuanYang VFD drive as do you. This isn't meant to imply these generate more or less electrical noise than any other drive - I don't know.
Three things I did as I built my CNC machine to stop the VFD's electrical noise from interfering with the electronics:
1) Made absolutely sure I did NOT daisy-chain electrical grounds.
2) Put VFD in it's own metal electrical enclosure.
3) Made sure cable from VFD to spindle was as far away as possible from other electrical cables (limit switch wiring, etc.)
I still suspected I had an occasional issue caused by electrical noise, so I placed noise suppression toroids
* On the power leads AT the VFD,
* On the power leads going to the spindle AT the VFD,
* and on the power leads going to a Programmable Logic Controller (AT the PLC) I use on my machine.
Because you will never know if you fixed the problem until it never returns (which hopefully is a long time), I think this is one of those situations where it isn't worth it to figure out EXACTLY what is causing the problem... it is easiest to address all the things that MIGHT be causing the problem and move on. OK, my logic here might be suspect (smile).
My toroids measure 18mm OD, 9mm ID, and 5mm thick.
Sorry, I don't have the specs, but the look a lot like
http://www.ebay.com/itm/7mm-Inner-Diameter-Ferrite-Ring-Iron-Toroid-Cores-Yellow-White-50PCS-LW-/181834403242?hash=item2a562ccdaa:g:iEYAAOSw4UtWTwGZ:
Not endorsing this seller, or the product, just that this is what mine LOOK like.
There are scholarly articles on what size, type, etc. to use.
Best of luck.