Its not RAM. It looks like other people have solved htis by turning down settings, by alt tabbing out and closing the overlay.exe and then continuing to play and even by forcing directx 8.1
What video card are you running and do you have any way to check your temperature upon a freeze? I am wondering since most people turned off their higher settings to solve it if it is just a result of your card overheating and finally failing at rendering. thus causing the process to be waiting for your card to respond but it wont render. Just a thought anyways.
You may want to search the steam forums, i havnt done that yet just some search engin stuff. First tink i thought befor eis earched was that it so[unded like card issue and overheating.