This is not a webapps issue, so please don't move it there. This is an issue with Flash, not with one specific website, as shown in this screenshot:
This issue appears to come and go with time, it often also manifests as 'stuttering' playback with audio corruption and strange artifacts on the frames. I tried to uninstall/reinstall Flash a while back when it first started to do this, and that worked briefly, then it came back. Now reinstalling it doesn't fix the problem, even temporarily.
It has been determined (thanks SilverbackNet) that disabling GPU acceleration in the Flash settings menu resolves this issue, but for obvious reasons, that leaves in question whether it is the GPU at fault. A quick run through FurMark yields an odd result:
The GTX260 never gets above 85C, when it gets to 85C the FurMark output stalls (only FurMark stalls, everything else is still accessible (so the GPU must still be functioning during these stalls) for about 5 seconds and the continues from where it would have been if it had never stopped (it's further round than the stalled image). I've seen it go above 85C in the past (albeit, in games, not in Furmark), but it has never stalled. As far as I'm aware, the GTX260's critical temperature is 105C, not 85C.