I have noticed this problem two times now. First time was when I watched "the first sign" cutscene in Diablo 3 (the intro video). I noticed that in some places, it felt like the framerate of the video itself was lower than it should have been. It didn't skip or stutter, it's like from the usual 24FPS that videos run, the video dropped to like 15FPS. Another game where I noticed this, was Deus Ex: Human Revolution. And it only happened during cinematics, not in in-game scenes.
My CPU was around 15-20% when watching the video. I also tried to watch the same video of Diablo 3 in Youtube, in 1080p but it seemed to run smoothly.
Windows 7 Pro 64bit
Intel i3 2100 3.1Ghz
4gb of ram
MSI HD6850 1GB
I don't think OC-ing my CPU would fix anything if the CPU usage was only 10-20%.
EDIT: My max monitor resolution is 1366x768. I turned the resolution down to 1024x768 (letterbox mode) and it seemed that the slow downs were gone. Quite a weird probem. I'm going to update my drivers from 12.3 to 12.4 and see if that helps.
Thank you for the link, I've seen it before but I don't really have any of these 1-2 sec delays in-game. The game itself runs pretty well on my PC. I'm going to try to update my drivers and see if I can run the video smoothly with fullscreen on and vsync on.