some games give this problem , my cpu i5 2500k@4.7 ghz / GTX 670 sli 4gb I cant find whats problem ? maybe take an i7 for use to Hyper Threading
That, but i would drop some settings here and there because by the looks of it that game is not so optimized.Or from another point of view 2x670 is super advanced for your system.
You didn't understand, the game either is unoptimized or the cpu is capping your frames per second.Probably both.