what do you mean by "maxed out sometimes" ? fps occasionally drops to 14? what is it the rest of the time?
what exactly is "everything turned on"?
when was last time you updated your drivers?
my guess is 580 doesn't need to go above 50% utilization to do SC2 @ 1680x1050
SC2 is a lot more intensive on the CPU when you maxing everything, so I think your phenom might be the culprit here
seems that cpu can only handle up to 30fps @1080p