So, I can be playing just about any game (WoW and Vindictus mostly.) And I can run just fine at 30fps or so depending on the game/settings. Then, all of a sudden, my frame rate drops down to like... 4-10 for a minute or two, then back up to a playable frame rate, around 30, and sometimes it will spike even upto 60, which is fantastic.
This does NOT just happen when there are lots of particles, lots of models, etc. The drop OR spike to 60 can happen during big boss encounters with lots of flashes and particles and shadows, or when I'm idle doing nothing in the middle of nowhere. i've tried shutting off background problems, but I'm still not sure whats up.
try turning off vsync and lower frames to render ahead to less than 3. During the low fps times does your hard drive seem to be running lots? Automatic updates running during your games or virus protection?
Might wanna try some kind of mem test program. If you ram is not functioning correctly it could be pushing game processes onto the HDD paige file, like when you run low on memory. Hence iam's question about HDD noise above.
I used to have that problem in oblivion while running a crap load of texture packs.