Just to add that I have the exact same problem using a Radeon X1800XT with the 8.31.5 fglrx driver and MythTV 0.20 from the repos. Playback is fine in all media players except MythTV's internal player. Cheers.
-- ATI Radeon AVIVO Video -> 'Blue' tv https://launchpad.net/bugs/73102 -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs