Can't watch HD videos

Hi, 

I can't watch hd videos on youtube on any browser.  They get choppy, laggy and eventually flash crashed.  All started more than a year ago but i still could use IE to do it. Now IE is useless too. I've searched a lot about this issue, but couldn't find the solution.  I'm using  Win 8.1 - 64 bit.  Flash player is updated (IE and plug-in based as well), graphic card driver is updated too (AMD Radeon HD 7310 Graphics), and so are the browsers.   I tried disabling hardware acceleration but it didn't work.  Cleared cache, deleted data on flash, and still nothing.  What other information do you need?
Thanks.