system
4
Streaming 1080p video through flash is no joke even on a i5. The nature video you are sampling is probably one of the more taxing ones b/c of the the content. If you tried to download it, the 13 minute video is 470 megs. Like pictures not all frames are equal, some require more compressed bytes to render so that would explain the fluctuations. The fact that flash is involved aggravate the matter.
Have you tried using the youtube html 5 player
http://www.youtube.com/html5
A lot better stability. I tried the video with flash and with html 5 player, with Avast enabled and disabled to see if Avast was contributing to the CPU fluctuations, but they were about the same.