Vertex Delta K8800 - Slippage

I have spotted very odd behaviour in my K8800 that I had not noticed before and I am not sure what the cause is, and I suspect it may be causing some of the problems I have been having with larger prints.

I was trying to diagnose some problems in a print. I cut the print down to the point where it has parked the head at X0 Y100 Z1, just before it purges the nozzle. I sent the print to the printer and watched its behaviour during levelling, everything seemed fine. The print completed, leaving the head at X0 Y100 Z1. About ten to fifteen seconds after the print finished, the north sled slipped downwards, causing the head to lower by about a mm, resting on the bed. Why??? This is about 75% - 80% reproducible. If this happens during a print, it could explain my gouging problems with larger models.

Is this a fault and how do I correct it please? I tighten all of the belts regularly, clean and lubricate.