Okay, i recently experienced this almsot non stop, especially when phase jumping ships if i was zoomed in
Im playing everything on max settings
i followed these steps and everything runs perfect now, at even better fps..... no settings changed.

I hope this may also work for you

1) Reboot into safe mode
When your computer is restarting and at loading of the bio (ie you see the motherboard screen before the loading of windows) hit f8 to have the startup option available, then choose safe mode

2) Uninstall the "nVidia driver" from the control panel > add remove programs

3) Go to (install directory):/windows/system32
and delete the file "nvlddmkm" i forgot the extension of the filetype, there will only be the one though

4) restart into safe mod !!!!! must enter into safemode!!!
when prompted about auto installing drivers say no!

5) install the following driver
http://downloads.guru3d.com/Forceware-173.65-Vista-beta-(32-bit)-download-1846.html

i am using a laptop wiht the following specs and no issues so far since doing that, before the change i was getting the crashed every 15-20 minutes!

Specs:
4GB PC5300 ram
2.33gHz intel core 2 duo (OC'd to 2.83 atm)
nVidia Geforce go 7950GTX 512mb
200GB 7200RPM SATAII hard drive

hope this helps out!