If you are having this problem with only one movie, then I'd say that it's a bad copy; try downloading a different copy.

When my computer is busy doing many things at once, it will sometimes play out of sync. Try again when nothing else is running.

For a novice user, you're doing pretty good to have read and followed the instructions re CODECs. (We LIKE people who first read before posting questions).
Let us know if my simple suggestions don't work. Somebody here will help you to figure it out.