Once thing is certain, from time to time the Adobe Flash Player becomes extremely unstable. It usually happens after a bigger update. Now is such a time, you are not the only one who has problems with flash 11.6.
1. Downgrade. Currently I'm using 11.2 with Firefox, that one is still stable and has much better performance than the current 11.6. If you're using Chrome, you can't downgrade easily, try something else.
2. Disable hardware acceleration. Right click in the middle of the video, select settings, and untick the hardware acceleration checkbox. You may need to reload the page, restart the browser or maybe even restart the computer to have the corruption disappear.
3. If there was a specific Catalyst version you used before, and it worked with the current flash version, you can downgrade drivers as well. Just make sure you do a clean install, downgrading over the top will not work.
#2 is the easiest and fastest, so it should be the first thing to try.
CPU: AMD Phenom II X4 810 @ 3250MHz | RAM: Kingmax 2x2GB DDR2 800 @ 833MHz| MoBo: MSI K9A2 CF v1.0 (BIOS: 1.D)| GPU: Asus HD 6850 1024MB (DirectCu) @ 850/1100MHz | Display: Chili Green Vision L24FHD | PSU: PC Power & Cooling Silencer 750 Quad | OS: MS Windows 95 x64 Edition
CPU: Pentium 4 Northwood S478 @ 3200MHz | RAM: 1,5GB DDR 400| MoBo: Gigabyte GA-8S661FXMP-RZ | GPU: Abit Geforce Ti 4200 | Display: Dimarson 19" CRT | PSU: Noname 400W | OS: MS Windows XP Pro, Fedora 18