same thing happenig to me (the BIG slowdown after a couple hours of playing)
i have a Pentium DualCore E5300 @ 2.60 4gb RAM, Windows 7 Professional x64 and a Geforce 9600GT 512 256bit
when the game stars i can play it on maximum settings and 16xQSAA anti-Aliasing and it runs fine. after 1-2 hours the framerate drops dramatically and during all the time the game was running windows says that processor load was 100% ay core 1 but the other one was idle most of the time the game was running...