Update - I think I may have resolved this fault, at least temporarily. I've just moved some wires around in the box a bit, to try and put a little space between the power feed from the PSUs to the drivers, and the signal wires to the drivers. They're only an inch or two apart but previously they were more or less sitting on each other.

So I plugged it all in again, and it's working perfectly. Ran it for a few minutes back & forth a lot on each axis, and no problems at all.
Every other time I've run it with everything in the PC case there's been an intermittent but quick to appear fault.

Not conclusive, but the one time it's worked perfectly is just after I've moved wires away from each other - don't think it's coincidence.

So I will try to buy some single core shielded wire to replace the current signal wires, and route them a bit better.
Also I'm going to make a ply shield with some foil on it (in case it helps) to fit over the power feeds o see if this helps too.

So happy to have it almost ready to use though. I've wanted to do this for years!