How are your outputs configured? I'll guess that framebuffer 3 is a DVI output to an external monitor. If it is, then have you tried a different monitor/cable combination? It could be the cable itself or the connection between the cable and monitor.
If you right click the output window on the right hand side (under the objects window in Layout mode), you can then click on "test output" and that will give you the option of displaying color bars.
Let me know if any of those work.
#XPression