---QUOTATION---
My gpu and core is overclocked, gpu at 680/1600 (shaders)/2000.
---END QUOTATION---
The overclock is your problem. It's running way too high for a very memory heavy game.
Your core is NOT running at 680/1600/2000. It's actually running at 684/1620/1998, due to stepping. Rivatuner would show you that. Learn to overclock properly.
Any problem you have ever in a video game can be blamed on an overclock if the base game works. All your have to due is reset the BIOS to default and run the mod and you would see that. Running 3DMark06 or anything of the like does not run your graphics card as hard as STALKER: Priboi does. I can run 3DMark06 triple tests with my GTS at those speeds, but it will error out in STALKER or Oblivion if I kept it on air cooling.
^ Overclock.net overclocker.