This is a problem that seems to have happened after the switch to the newest version of Flash (11,2,202,228). Anyways, I've been noticing videos are fine as long as I keep them in view but if I scroll down on a page, like on Youtube for instance, or browse another window while a video is playing, when I come back to it it will start going crazy fastforwarding-like until it gets to what is supposed to be happening at this moment. I've noticed it on Youtube and also on Skype, the sound usually will continue to play if I go to another window (although there have been a few cut outs) just the video is screwed up when I return to the video, which runs on Flash. Also I used both Firefox and IE, and this happens in both, so it's not a browser problem.
I've thought that this could potentially be a virus however I can't imagine any virus would just affect Flash videos and not affect anything else, considering my computer seems to be running fine otherwise and I've already run quite a few antivirus/antimalware programs, which came up with nothing.
this looks to be a known issue. the internal bug (#bug=3149726) was deferred in the 11.2 brannan release but it is resolved in our dev builds in 11.3 cyril. we have a private cyril beta out and should be opening for the general public soon.
actually, the public beta went live yesterday. you can try it here if you'd like: http://labs.adobe.com/downloads/flashplayer11-3.html