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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:

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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:

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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.

Any ideas?

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a whole lot of info, yet you fail to mention what browser you are using nor what OS you are running on. – Nifle Jan 8 '11 at 2:29
Well, I thought the screenshot made it obvious (browser is clearly displayed as Mozilla Firefox, green forward/back buttons means XP). To be more specific, XP SP3 and Mozilla/5.0 (Windows; U; Windows NT 5.1; en-GB; rv:1.9.2.13) Gecko/20101203 Firefox/3.6.13. – Matthieu Cartier Jan 8 '11 at 10:06
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To me it looks like corrupted video card acceleration. Is your card overclocked, or badly ventilated and clogged up? Are you drivers up to date? To test, right click into the Flash player settings and turn off hardware acceleration, on the first tab.

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That is very intriguing. That does indeed fix the issue (albeit the fix is not ideal). The card is overclocked from a BIOS mod I did about a year ago, but I monitor the temps and voltages, and they're all okay. The drivers are the latest 32-bit XP Forceware drivers from nVidia's website -- but I don't think it's that, because it's been fine for ages. Have you seen this issue before? – Matthieu Cartier Jan 8 '11 at 2:23
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Oh yeah, sometimes green screens or random stripes instead of video too. Maybe the card's just getting old and developing tin whiskers or cumulative tiny electrical shocks? I assume you do gaming since you monitor it, but it might be time to load it with the heaviest benchmarks you can get it to run it and see if it's stable with correct output. – SilverbackNet Jan 8 '11 at 2:35
Before I forget, you could try an older (or perhaps newer/beta) version of Flash player as well, they're still out there. – SilverbackNet Jan 8 '11 at 2:36
Yes, I do regular gaming (play competitively) -- never had a problem in-game that was caused by the card. I regularly bench this card just because I'm a bit of a nerd for that, nevertheless I'll load it for an hour or so with Furmark and see what happens. If that doesn't turn up anything, I'll try a different version of Flash. Thanks, these are probably the steps I would have recommended to anyone else, but for some reason when it's your problem it seems you don't think logically... :) – Matthieu Cartier Jan 8 '11 at 10:10
Okay, I've just ran it through FurMark and found something a bit interesting, adding it to the original post now. – Matthieu Cartier Jan 8 '11 at 10:25
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I'm thinking when you say it happens during stutter, that it's a bandwidth issue and the video codec updates sections of the screen by saying "this color difference from what it was". then if you miss several frames the video goes funny like that. i used to have this problem when converting from ogv, but i sorted it out. As far as correcting the streaming issue from your end, I'm not sure there's a lot you can do about it. The provider would need to use a more robust codec or keyframe it more frequently. That way, every X seconds the whole frame is updated instead of saying "change color for region by X".

At least for vimeo, you can download the whole video and watch it locally. For youtube idk.

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Nope, the stuttering occurs with similar screen artifacts, and occurs when it is fully buffered. For the record, the one on the left is one of my videos -- the codec I use is irrelevant, as YouTube does a full lossy recompression into whatever format they use. Either way, this is not a "normal" issue that can be explained simply by lack of bandwidth. Thanks for trying though. – Matthieu Cartier Jan 8 '11 at 2:19
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