http://www.patches-scrolls.de/

Have you noticed heavy HDD activity during slowdown period?

What's the size of swap file? What OS?

Check processes and memory usage via Task Manager

just to eliminate memory leakage.

Also scan your system for malware.

There is always a sweet spot.

Little contribution:

Disabling Catalyst A.I. helped me with similar issues.

Also disable onborad sound card in BIOS as asphoRe suggested.

If that does not help, try with forcing DX9 via settings.ini in Documents/BFBC2 folder.

Have you disabled SpeedStep and lower power consumption options in BIOS?

CPU Enhanced Halt (C1E)

CPU EIST Function

C.I.A.2

BTW, what Mobo, CPU and RAM you have in rig?