Comment 17 for bug 388456

Try change:

  UseLegacyModeSwitch to FALSE

in via_driver.c file:

        case VIA_PM800:
            pVia->VideoEngine = VIDEO_ENGINE_CME;
            pVia->UseLegacyModeSwitch? = FALSE;
            break;

then compile and install.

Attach Xorg.0.log file