Metro 2033 lagging FPS

Is anyone else having a problem with playing metro 2033 on high settings? i have turned the AA off and tessellation is down and it is smooth most of the general game play but when i throw a bomb or grenade it slows right down.

I have a:
CPU - I7 950 (Clock speed)
Motherboard - Gigabyte X58 UD9
Memory - Corsair 12GB (6x XMS3 2GB) PC-12800 (1600MHz) XMS DDR3
Graphics - 2x HIS ATI Radeon HD5850 1GB iCoolerV CrossfireX
HDD - OCZ Vertex 2 60GB E Series SSD, Seagate Barracuda 1.5TB
Monitor - Samsung BX2450 24inch Widescreen LED Monitor (1600 x 1050)
Operating System - Windows 7 Pro 64-bit OEM

I am running it at 1600 x 1200

Turn Physx off and the game will running fine

really easy as that? will try that thanks

Hi DKRON
your system spec is excellent ,
but the slow down in the game when you throw a grenade is happening because you dont have a hardware to process the heavy physx of nvidia so you need to choose one of the three options here :
1 - turn physx off as i told you.
2 - if you like physx and you don't want to turn it off , then you need to buy a nvidia graphics card for physx like gt 240 , gts 250 ,gtx 260 or better.
3-if you dont want to buy a graphics card for physx try to overclocking your cpu this
could help you to have better fps.
and good luck

Thanks it cost me alot of money to build my computer, it is a pain that amd cards don't work with physx but from the reviews on techspot even the gtx590 had trouble running it high, i might be swapping up to Crossfire 6970's soon