I had this exact same problem when working in 1080i. I tried 3 different Io HD boxes, chaneged driver software, changed firewire cables and ports with no success. Later had the same problem on a brand new Mac Pro. I set my field dominace to NONE and the flicker stoped. It does make some of the livetype graphics a touch blury becuase they are inteneded to be interlaced but thats a better option than the video shifting.