S.T.A.L.K.E.R: Clear Sky Performance Problem

Hi, this is my first thread and post at this forum so I'm not sure if there were specefic subforums for STALKER questions... Anyways, here we go:Yesterday I bought S.T.A.L.K.E.R: Clear Sky and was able to play it at reasonable FPS at maximum graphics with the resolution of 1400x900. I played it a few hours yesterday and a few hours this morning. But an hour ago or so I was hit by a wall of lag when I tried to play the game. I haven't changed any settnings or anything similiar. All I had been doing was playing some Fallout 3 in between. ( which showed no sign of problem ) Now the game is totally unplayable. I used to have 50 FPS but now it's down to 15 for no reason! This has rendered the game totally unplayable and I have no idea what could have caused it... I've tried asking at the official forums without success so now I'm laying my hope in your hands. Any hints on how to solve this issue are very much appreciated.( Note, other games still run fine. It's just STALKER: Clear Sky. Also I excuse for my bad formulation of words and sentences... English is not my native language. :) ) S.T.A.L.K.E.R: Clear Sky Performance Problem
Actually nevermind... I just tried launching L4D and Fallout 3 and it appears BOTH of them have horrible FPS all of the sudden... used to have 75 in F3 and 60 in L4D. :S