I'm stumped by this one. I have a Quad-Core i7 15" MBP with 8GB of RAM. It has a 750GB Western Digital Scorpio HD running at 5400RPM. Whenever I play a song that has a moderate to aggressive beat and base, the song starts stuttering. It will just pause for about half a second numerous times throughout the song. This happens regardless of whether I play the song in iTunes or Quicktime, and happens with any song that meets the criteria. While the computer is struggling with the song, the machine's overall performance completely tanks. Spinning beach balls left and right. If I lower the volume to half, or lower, the problem goes away. My buddy has this machine, but with an SSD drive. His has no problem with any kind of music. My theory is that the volume causes enough vibration in the unibody frame to trip the Sudden Motion Sensor, causing the HD to park its head for a split second. Does that sound likely? Any other ideas?