I think you're seeing one of the following:
- Vsync hiccups
- Tiny garbage collection pauses
- Low-resolution timer artefacts (fixed in Firefox 15+, Chrome 21+)
There's not much we can do about any of these part from wait for the browser improvements. I think these pauses are typical but show up more noticably in your example because it uses large, fast moving brightly coloured sprites.
It seemed better in Chrome 21 (Canary). Since there's nothing for us to act on other than waiting for browsers, I'm going to close this bug.