Currently use a Intel Core 2 Quad QX9650 EE with a couple of 8800GTX's, The problem I had was with a monaco track..Major slowdown, same at Sebring. Normally have well over 120+ fps @ 1680x1050 with everything cranked up and view distance at very far but was getting 34-40 fps..some tracks are just not as well optimised as the std gtr tracks.
Major thing that fixed this was to put shadows and shadow quality down to medium.
I never get stuttering at all even when I ran a AMD x2 setup with a single x1950xtx So maybe data loading related also. I Have dual Raptors in a Raid 0 config so maybe it was your hardrive holding the system back with that wacking great processor you have or possible cacheing probs? Computers are funny old things..could be all manor of things...driver conflicts that show up months later after installs, low temp memory or low resources..IRQ conflicts...I could go on but wont lol
Actually found that anistropic filtering at 2x can be much faster than triple buffering. Just a case of matching the right settings to your hardware...takes a bit of time but its worth it.
GL anyone thats having probs
Ross