How can I improve my RB corps?

Right now I have Steven Jackson, Trent Richardson, Doug Martin, Deangelo Williams and Daryl Richardson. Jackson hasn't gotten me jack and Trent is boom or bust, and more often than not its bust. Doug is my only consistent RB, what trade should I attempt involving ONLY RB's? It's a 12 team, standard scoring league, I've been eyeing players like Ryan Mathews, Frank Gore and MJD.