This seems to affect all videos, I have tested on two Win 7 64 bit systems with IE8. The Silverlight was the latest offered (I installed it into a clean system).

repro:

play http://channel9.msdn.com/Events/TechEd/Europe/2012/SIA302

Go fullscreen, seek to 5 minutes by clicking on the bar and let it play. I haven't paid attention to if it is immediately stuck or gets stuck later.