try to uncheck the setting in the Blackmagic Design Control Panel "remove field jitter when video is paused" and see if you dont get a better picture. Just did here on another computer where we had this exact same problem.
Is the resolution of Video Preview supposed to match the comp window? I am using a Blackmagic card and even when my comp is set to full, the aliasing in the Video Preview makes it look about half what it should. Just wondering….