Originally Posted by WeaselWords
I don't remember this problem being present previous to the recent Linux kernel update, but I'm not sure if those two things have anything to do with each other.
Nope, it has nothing to do with that. This is a known issue, though, which is caused by a broken Flash plugin. It occurs due to Flash taking assumptions about certain behaviour when all relevant specifications say you really shouldn't do this. Through pure luck, this worked in the past. Now some Intel developers wrote a performance enhancement patch for glibc which changed that behaviour (which you must not rely on). Adobe did and so their software went up in flames.
They are aware of the issue and are committed to fix it. This may take some time, though. Due to flash's proprietary nature, Fedora developers are not able to (and not legally allowed to) fix it.
You are not expieriencing it the whole time because the broken behaviour in flash seems to be restricted to certain codecs (e.g. MP3) while others are not effected. Youtube does not use MP3 for audio with the exception of the lowest quality vids (and advertisements, as it seems). It only occurs in F14 because previous versions didn't incorporate the new glibc code.
You can track the state of the bug at Adobe's public bugtracker.