I made this simple light track with no Explosions or external views to get a feel for stuttering and FPS. It has most of what you described.
I average 38-42fps throughout with minor stuttering, max of 58 and min of 18 (momentarily in one part). On the final run into sun over land for last 30sec between 42-46 fps
Settings: 1920x1080
No AA
AC models High, Buildings low, Land High, Forest Low,Vis FX High, Damage models High, Building amount low, Shading medium, Grass Shadows and roads all off.
http://www.mediafire.com/file/nsrgcc...tresstest4.zip
This was post beta patch. Windows 7. NV drivers 267.24. Std IDE Sata drive. Oh and the weird thing is that EVGA precision (diagnostic tool) is showing both GPU's working. But when I try on my SSD drive only one works....Go figure.