Problems with the OSD (menu appears and disappears randomly, or flashes 'locked and unlocked' on and off - Or other faults) are caused by the buttons themselves.
These Philips monitors use poor quality buttons that after 2 or 3 years wear out and go semi-short circuit. You can measure them with a meter.
The permanent fix would be to replace the button (open up the monitor, desolder, and then resolder) - A quicker / less exact fix would be to just pull the cable off the button panel inside and do without the controls altogether - again, involving opening the monitor up.