I've tried just about everything mentioned on this thread with no success. Once my CPU undergoes significant load, the various problems start surfacing.

But for now, I can live with using one core. I can manage to play 720p with Mplayer OSX 1.0 RC2 with only minor instances of lagging. The lag was a concern for me using a single core but I realized I wasn't using the RC2 version of the player.