Has there been some discreet update on your OS or something like that? You could check whether there's a process running in the background while you're playing as well; sometimes, games and other software like to download updates without telling the user first, which results in a catastrophic drop o...

I ran into the very same issue, came here to find a solution, and yes, it's solved by installing a "clean" version of DirectX. I didn't expect it to be that simple..