How can I fix my under-performing team?

You've just had some bad luck. You don't need to do anything radical. But try trading Barber and Houshmandzadeh for a RB (maybe Brandon Jacobs + another WR). Barber isn't really much better than the other two guys on his team and he can't stay healthy. But people still love him so he's got value. Housh is just not that good (but he's still a big name to some people). Put in Davis instead of Shockey, and Lynch instead of Julius Jones (Davis is the only thing SF has going right now, and you can't put all your eggs in one basket with NO recievers. I guarantee you J.Jones is going to fade and Lynch will come on.). You might have to cut your losses with Westbrook and accept whatever offer comes your way.