YouTube re-encode anything you upload so it's not really going to be a final cut issue. Either something's messed up on the YouTube ends yen they encoded the clips or it's a playback issue on that particular Mac.
YouTube are, of course, free to change their encoding at any time, and maybe this time it's not for the better.
Have you tried uploading again? Maybe second time lucky.
(For some reason I can't edit my own posts so apologies in advance for the stupid mistakes and bad English that I can't go back and fix)
Jeff Kirkland | Video Producer & Cinematographer
Melbourne, Australia | Twitter: @jeffkirkland