I have a 2-year old MacBook that works perfectly. I'm running Snow Leopard and I use Safari as my web browser. Whenever I watch videos on YouTube, the audio and video are out-of-sync. At the start of the video, both are in-sync, but as you get near the end, they are way out-of-sync. After an hour, the video and audio will be off by 5 or 6 or so seconds. I tried switching web browsers to Mozilla Firefox, but the same thing happens. I have TechToolPro and totally optimized the drive, and it still happens with both browsers. What is causing this ?