|Have the same problem regularly with an Acer notebook and a Hyundai LCD.|
Changing resolutions or switching from Clone to Extend will always show the desktop for 2 seconds, then it goes black again. Even the OSD won't come up.
Solution here: In the ATI Catalyst Control Center, go to the "Desktops & Displays" section. Select the second display and deactivate it (Right-click on Icon, then Deactivate).
After that, set it to the desired mode (e.g. Extend). (Again, Right click, then "Extend").
This works for me every time.