Comment 80 for bug 312756

Have you guys tried specifying BusID in your xorg.conf?

      BusID "bus-id"
              This specifies the bus location of the graphics card. For PCI/AGP cards, the bus-id
              string has the form PCI:bus:device:function (e.g., "PCI:1:0:0" might be appropriate
              for an AGP card). This field is usually optional in single-head configurations when
              using the primary graphics card. In multi-head configurations, or when using a sec-
              ondary graphics card in a single-head configuration, this entry is mandatory. Its
              main purpose is to make an unambiguous connection between the device section and the
              hardware it is representing. This information can usually be found by running the
              pciaccess tool scanpci.