The machine is pristine now so have ruled out everything else. Luckily Yarnmonger on Ravelry reminded me that Richard Croucher can fix these things (and he's in the UK) - otherwise a new cable is $299 CAD plus postage and import duty. Ouch!
On the plus side, my KG93 garter carriage has chugged away happily for 470 odd rows and nary a dropped stitch. There are some errors on the right hand side because I put the turnaround cam a little too far in and the pattern hasn't quite reset in a few places, but it's hardly noticeable. I'm hoping I'll have time to finish it off tonight.