Howard / Mike: The cards are sync'd together using word clock. I've also tried syncing via SPDIF - both work fine. The cards seem to sync together fine, the key issue comes when putting everything together with a QT movie. When there is just a heavy audio only session, it comes up with the normal range of "errors" that you'd expect - Core Audio Overload / Disk too slow etc. Slot wise, they are in slots 2 and 3 immediately after the graphics card. One thing that has eased the system head load up a little, is using the "Each" card synchronisation option (on the Maudio control panel) which (apparently after much digging from Maudio) slightly reduces the load on the system. One thing I am going to try in the morning, is running the video from another different drive. After a load of tests today, I think that it is just starting to be on the very edge of what my machine can realistically do - and so the cracks start to show! Thanks for you'd advice. Grant