Yeah I had that issue once I got to the boss fight. It's wierd, but the fix is fairly simple. In your desktop shortcut, open the properties and the line where it has the target, at the end add:
Or, if you bought it on steam, you can open the properties and then there's advanced settings or something along those lines, and you type that into the box.
For whatever reason there's a memory leak. Having it in dx9 will fix it. But just keep in mind if you go back to replay the game, delete that or else you'll be stuck at dx9... still looks good, but yeah.