I’m having the same issue running from a thunderbolt LaCie, (MacBook Pro 2011, 2.3GHz Intel Core i7 8GB 1333 MHz DDR3) 4 camera multicam (1hour long). Enabled Multiclip Playback in the RT settings, set to ‘Open’ (I have done a few SD multicam edits before without a problem) Files for this edit have now been converted to proRes. Its all fine until at first but as I get further into the edit then I encounter the same issue as David. My only conclusion is that I’m pushing my system to hard. I’ve spoken to a friend who had the same problem with FCP7 but more luck with X. I have to use 7 as I email the project back to my client who relinks media and adds graphics before export.