Is it when flying in a certain area? I sometimes find some sceneries slow things down.
Also you don't need to have your FPS running above 25FPS as that is all the human eye can register and above that you're just wasting processing power.

Well, I've known mine to, when running right on the very limit between acceptable framerate and as high scenery as I can go, to occaisionally make quick jumps down to 15 for half a second and shoot back up.  Though I do have a few other hints that may help your computer.

A few small ones you'll probably never notice missing are by decreasing the fs's screen resolution and color resolution as low as it can go.  Also turn off anti-aliasing and all the light bloom and lens flare effects.  Also take the filtering level down to trilinear, or even better, bilinear.

The things that REALLY kill my system are these:
1.  Autogen/dynamic scenery and other 3D objects.  The most my system can ever handle is normal settings.
2.  Clouds. What you can do for these.  I actually found a solution for clouds, where you don't have to have ust a clear sky, and you also get better framerates than if you did just have a clear sky.  Where ever you would want clouds, just reduce the visibility level at that height to much lower setting.  Pretty much all you ever need for clear weather is 10 miles.  By lowering the visibility, you actually greatly increase your frame rate for two reasons:  1, it doesn't have to draw the clouds, and 2, it doesn't have to draw things so far off that you won't even care about them.  It also can add a bit of realsm t any flight.



Hope these help.