Can you give me an example URL where this is occurring? I'll forward this along to our video team. In addition, I'd like to recommend you open a new bug on this over at bugbase.adobe.com. This FAQ will detail what information we'll need from you.
Can you send me (firstname.lastname@example.org) a copy of your dxdiag report? Also, I will need an exact url from youtube. I know that seems silly, but some videos are different than others and we'd like to make sure there is no confusion or mistakes when trying to reproduce this problem.
Here's a faq that details what steps we'd like people to take to troubleshoot and how to officially report video problems to us:
I'm also having this problem! After updating do 11.2, youtube videos appear exactly as seen on Patrick's screenshot (the "player" looks pixelized and if the video has text, it appears with "steps" along the lines - screenshot: http://i.imgur.com/qysba.png - Also, now when I play videos in full HD, there are dropped frames! It never happened before!)
I'm running Windows 7 SP1 X64, and the problem occurs on Firefox, Palemoon and Chrome, but not on Internet Explorer and Opera (I don't have any idea why!).
When I was using Flash Player 11.1, youtube videos played smoothly and it showed "Accelerated Video Rendering and Accelerated Video Decoding" in fullscreen - now, it shows "Software Video Rendering and Accelerated Video Decoding".
My system specs:
Intel P6100 Dual core 2.0 Ghz
Hybrid graphic card (AMD switchable graphics - Radeon 5470 + Intel Integrated Graphics). Intel is using an early 2011 driver version because it can't be updated, but AMD is running with the latest driver version released - and I'm always using AMD.
320Gb hard drive
4gb Ram DDR3
Is there anything that may be causing this?
[My english might have some mistakes, sorry]
Firefox button -> Extras -> Extensions -> Disable (or remove) Youtube High Definition 1.9.
Note: Other extensions like thumbnail preview for facebook or other Youtube High def plugins might trigger the issue. You should disable one by one until you find which extension if causing the problem.