I have experienced the same problem with this monitor:
i use only analog VGA input to the monitor, the monitor switches inputs automatically, some times rapidly and sometimes slowly.The monitor will switch inputs even if it not conneccted to any inputs. I am thinking the video processor/controller is broken because the change input button doesnt work either to change back to your preffered input. could it be the "Switch Inputs" button?
I think the problem started with the use of the USB ports on the monitor, I bought it in 2005, is it still under warranty?