i knew that problem... have you tried to download it a 2nd time? if yes may you should try to fix the files... you can do that by rightclicking on the link in Steam and choose properties.. then you choose the head lokal data, and first you proof the integrity of the game... if it will find no error, you defrag...
I'm not some kind of expert of this, but try same as Sinner asked ... try to update your drivers might help Your reasoning is excellent -- it's only your basic assumptions that are wrong.
might help