G320X going into fault on 4th axis

I've been adding a 4th (rotary) axis to my CNC machine, but the Gecko G320X running this axis will go into fault sometime between 30 and 90 minutes of running. Despite a lot of investigation I've yet to work out why, and could do with some help.

The g-code is generated by DeskProto, and spins from 0 to 360 degrees back to 0 degrees; whilst raising and lowering the Z axis, and stepping forward on X; though as I've slowly removed variables as part of the fault finding I've now gotten down to just the 4th axis running, with no spindle (i.e. no actual cutting).

I've put a camera on the G320X to check its behaviour. Even a short clip is too big for an attachment (~1.7MB) so I've uploaded it to this link

The in-pos light flickers during running, but when it faults it appears to go straight to the fault condition (no warn).

Checking the G320X manual, I've been going over the possible fault conditions:

Short-circuit: not impossible, but I can't see any loose wiring, and re-enabling the G320X brings it straight out of fault
No encoder attached: I'm using an AMT102-V, with their CUI-3131-6FT lead. Connections at both ends look good
POSITION ERROR exceeds the set following error limit. This condition can have several causes:

1) The loop settings are severely under-damped and the motor breaks out into oscillation. I've tuned by ear rather than an oscilloscope, but the motor sounds stable
2) Excessive motor load due to acceleration or workload. When it trips out it doesn't appear to be doing anything unusual, and I can rapid all 4 axes of the machine without problems
3) The speed command in excess of what the motor can deliver. Rapids are fine, and the drive faults at random points in the g-code, so I assume the code is OK
4) The current LIMIT is set too low. Limit pot set fully CCW (for 20A limit)
5) The power supply current is insufficient for the demand. Should be good enough for all motors (I can rapid all 4 simultaneously without problems)
6) The power supply voltage is below 18 VDC. 24VDC
7) The motor is wired backwards, is broken or disconnected. I guess not?
8) Encoder failure. Possible - but it is brand new.

There are no obstructions for the 4th axis, and it's smooth enough that I can move it by turning the motor shaft with a thumb and forefinger. The motor body and the rest of the machine are grounded to my mains supply. All dip switches on the G320X are set to their defaults (as per the manual). When I'm doing the testing there's nothing electrically noisy running the same area.

I don't believe it's a fault with the G320X (it's brand new), and I've swapped some kit around and get the same behaviour with two different G320 (the older non-X) drives (it's always the drive on the 4th axis that faults) - but something is upsetting the Gecko and I'm running out of ideas, so any help would be appreciated!