The Saints Row series does not have a good history when it comes to the holy PC. The port of Saints Row 2 was a buggy mess that could rightly be called an abomination. Saints Row the Third is better, but issues involving stutter and performance problems have been dogging the murder simulator since release. Volition has stated in the post on their forums that they have pinpointed the issues and are working with hardware manufacturers to fix the problem. The Saints Row forums has the story.
Meanwhile, if you’re running Saints Row on the PC, I highly suggest checking out the “Gentlemen of the Row” mod for Saints Row 2 and “Gentlemen of Steelport” for Saints Row the Third. These mods cure many of the bugs that haunt the games, while offering a buffet of tasty, tasty goodies like unlocking the character creation sliders or giving you a gun that blasts targets into low-earth orbit.
Highly recommended.