I seem to have found a workaround for this issue which I assume is a fault that has cropped up on the desk, but I would appreciate it if someone can suggest a way of getting rid of the issue entirely, and I could go back to my old setup. I have 2 IF-DM/AD cards and I was using these and the on board ADAT ports to run 24 channels to and from my RME RayDAT card in a mac. Recently I've been losing sync between the RayDAT and the onboard ADAT ports on the desk. The connection to the IF-DM/AD cards was fine. The desk was acting as master clock with RayDAT as slave. I've tried swapping connections/cables, and running with master clock from the Raydat but the problem always seems to come back to the onboard ADAT on the DM4800. I'm now working around this by using my IF-TAD ADAT to TDIF convertor box to connect with the desk using one of the onboard TDIF connectors (good old TDIF!). With the RayDAT as master clock, the IF-TAD syncs over ADAT and provides a wordclock output (BNC) which the desk is happy to sync to.