Re: Weird issues with Episode 6 and mp4

Just in case, have you considered updating to 10.6.8 specifically using Apple's Combo Updater (not Software Update). If it's OS related it may clear it up. Since it seem specific to your config (it's not a common problem) it may be some odd thing running interference.
Also if you have Time Machine running and/or any virus checkers turn that off.
Another thing to try is to create a New User Admin, delete all login items for that account, reboot and login in to the new admin account and test there. That would determine if there's a user specific issues vs a system wide issue.