You could try the xf86-video-ati extension and see what happens, perhaps your hardware needs both drivers to be present?
I see your mode is found: [ 372.470] (II) VESA(0): Modeline "1366x768"x0.0 85.50 1366 1436 1579 1792 768 771 774 798 +hsync +vsync (47.7 kHz eP)