Things to consider........
Have you enough RAM?
Is anything you're using "leaking" memory (this usually involves a process asking for new memory for new tasks, but never telling windows it's not using the stuff it just finished with, so you end up with a bunch of RAM allocated which is not being used, but cannot be freed for other processes).
Do you have a dynamic swapfile? if so, windows will often decide to resize it, which can lag your machine terribly...but usually only for short periods.
Most of the above will also cause the drive light to hammer away all the time.
It may also be a background task choking the system, minimise the number of autostarting tasks using MSconfig.
I have noticed an issue with (I have assumed) the NV drivers, where they....it's hard to explain..............
Using MSTS, if I take the external "back of train" view, I've set a VERY tight field of view and the camera starts out pointing down at the train with only a bit of rail as background....this view ALWAYS ALWAYS hits 60fps when the machine is freshly started, never does it waiver in the least. If however MSTS exits badly, OR if I've been playing a few other games....this will drop to 59, and then progressively, through further MSTS bailouts (including clean exits after failed activities), and running of other games, to 58, 57 56 and as low on one occassion as 43!! Other games and benches are effected too, not just MSTS, it's just my "yardstick".
This does not seem performance related, because that view in MSTS is a doddle and the "new" framerate limit is so static, it's almost like some sort of frame timing glitch.
Beyond that, I have no clue, but it's a more subtle effect that the memory related ones I mentioned initially, I thought I'd describe it to see if you can observe something simillar (you need a "guaranteed, no questions asked, ever" 60fps game/view/scene to measure with).
