I play the game on an i5/750 running at 4.2GHz with a 5770 at 1920x1080 with high settings and it never stutters or lags ... so I'm pretty sure it isn't your hardware.
If you're running the latest drivers, you can cross that off the list as well, since I'm running them without any problems.
Since it is the explosions / track mangling that is causing issues, I suspect it might have something to do with the Havok physics engine that the game uses during 'power plays'. Do you have anything else running in the background that might cause the CPU to hiccup?
A little off topic, but nice overclock on that 750 Jip!