I had this problem too and I just fixed it. My problem is that my processor was over heating. I was cleaning my PC about a week a ago and didn't plug the fan back in for the processor and installed my bfg gtx 260 and was hyped to use it but when I played a game I was getting the same issue as you are. I thought it was my card and I went back to check things and turns out it was just the fan unplugged.
So...maybe another part on your computer is overheating. Or maybe your mobo is having issues with transfer speeds with your card and you might need to get a newer one.
You can Google "random lag spike" and it'll show a whole bunch of hits where other people are having the same issue. Hopefully you can find your solution.
hey im having a similar problem. i have a 4850 x2 2gb wich is like godly.. i get 100 fps in cs1.6 pub servers. but in cs1.6 zombie server with 32 man. i experice lowwww fps it even got down to 40-60. i get 100 fps sometimes in zombie plz help.
HMMM you've got 4g of ram,,,get rid of the swapfile,I only have 2g and I got rid of that soul sucker long time ago you should be running at least a 500w psu from a reliable source and of course you do defrag regularly http://www.majorgeeks.com/Auslogics_Disk_Defrag_d5266.h...
fastest defrager in the west,does not consolidate tho..
Ok you won't believe how stupid I feel now.
It appears to be indeed an overheat issue. I was really impressed at how the CPU cools down just by switching tasks while playing the game, to check the sensors.
I opened my case and stuck a big fan beside it and voilá! Perfect smooth gaming.