I would turn off crossfire to see if it fixes it, then you know its a crossfire issue.
After that I would see if its a cpu affinity issue, I know Race Driver Grid had some issue with my i7 on my laptop because it appears to the game as a "8 core" cpu and they only had it coded for up to 4 cores. Maybe NFSS has the same issue?
Does the same thing with crossfire disabled or one GPU. I have been talking back and forth with EA and they had me send them my dxdialog this shows all of my specs and everything else.
So far they have had be do a cleanboot with msservices disabled. That did not fix anything. They had me create a new admin user account, that did not work either.
I had this same problem with my Q6700 assuming that it would support that. But I just recently upgraded, and when I did I reformatted, reinstalled the game on steam, and having the same issues.
So far EA is blaming it on ATI issue and showed me threads on there forum where ATI users have a lot of problems with the game so. Now I am just waiting for them to get back to me.