[HELP] Bug at "Level 4"

[smokingman]

I am arrived at the level called "lv 4 - stronghold" (in italian version is "lv 4 - roccaforte"), but the frame rate go down at 2/3 fps.
The levels 1-2-3 i played on 35/40 fps, but the lv 4 is unplayable.
It's a bug becous putting down all graphic settings i dont resolve the problem.
On my pc i played in full graphic quality "Painkiller" and the second chapter "Painkiller Overdose", on 60 fps (and all graphic settings on "high") without problems.
Wath I can do for play "Necrovision" ?
I installed the 1.1 official patch, but the problem remain.
I hope a lot in a your answer, plz help me ^^

CPU: Athlon +3800 x1
GPU: GeFOrce 7900gt 512mb
RAM: 2gb
APU: Sound Bluster Creative Fatal1ty fps DeLUxe

SmoKingMan