(In reply to comment #4)
> (In reply to comment #3)
> > AFAICT Twitch requires Flash; what Flash plugin are you using in what
> > browser?
> 
> Yes, Twitch, like YouTube, requires Flash. I use the plugin installed by
> flashplugin-nonfree and its update-flashplugin-nonfree script. It's the 64
> bit version. I check the version number later again. But it was recently
> updated.

Sorry, forgot to name the browser: Iceweasel 31.0-3 (aka Firefox 31.0).

Just to note this again: other video pages like YouTube don't show the problem
Twitch has. And it's also only giving these spikes, when you try to watch a
recorded broadcast, not for live streams.