iTunes 11 Stuttering During Playback
Buried deep in the comments are these answers to solve the riddle of the stuttering iTunes playback on Mountain Lion:


  I've just remembered that I have FileVault enabled, and as the crashes / skips happen when the computer is performing file ops, I think turning FV off might help - decryping my hard drive now... – George Pearce Jan 12 at 14:34  
  
  Update: Turning off filevault and decrypting my hard drive appears to have entirely solved the problem. – George Pearce Jan 12 at 19:20