I saw something unusual on your Task Manager screen; Windows was using quite a LOT of your RAM as kernel memory, over 916 MB of it nonpaged, which is HIGHLY unusual.
As a comparison, my own Windows 7 x64 system which has an uptime of 3 days and 8 hours only has a usage of about 271 MB of Paged and 45 MB Nonpaged Kernal memory.
Glad to know restarting your PC solved the problem though. And yes, i suspect your loss of FPS is related to all the paging windows had to do for the game because it could not be loaded to RAM.