Videos going black after half second / plug-in for Chrome

btw - I couldn't find a category for videos under software, and YouTube is kind of a social network, but not really, so Social Networking is probably the best place for putting this.

So, I was just watching videos as I always am, and all of a sudden, every video I watch, I see it for about a 1/2 second and then it just goes black and I have to go back to the beginning.

It only started doing this about 10 minutes ago (as of posting this), it only happens on youtube, and it is with every video.

Not sure if it matters but the computer is a Compaq Presario CQ5320F which I have finally replaced after 5 years with a newly built computer (FX-6300 and R9 270 2GB) but my new one still has a few problems I haven't had the time to fix yet.

I have never done any alterations to the hardware or software, except for installing all of my things obviously. hardware is all stock.

Doesn't do this on my newly built computer, that's good
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I know this isn't related to what's above but does anyone know of a plug-in/add-on for Chrome that automatically changes videos to 720p and (optionally) switches annotations off as well so I don't have to change those myself every time I watch a video? Not that big of a deal but its kind of annoying. I had a plug-in/addon for Firefox that did the same thing but since then I've switched to Chrome and might possibly go back because I kind of liked some of the things that Firefox offers.