As the lamp ages, the shape of the two anodes may change from sharp points to a flattened state. When this occurs, the arc jumps across the gap from varying points. When the anodes are flat the arc will be lost and then reestablished. This is seen as image flicker. The length of time for the anodes to flatten, over which flicker may occur varies considerably and unpredictably. This behavior is inherent to UHP lamps.
It is possible to recondition a lamp to have sharper points once they are flattened. Professional projectors like Christie have a mode for this that will pulse the lamp and restore it in minutes to hours. Consumer units do not have this option, but they can sometimes restore a lamp by operating it in high lamp mode for many hours. I believe some consumer projectors like Sony and Sim2 have lamp driver circuitry in them to recondition the lamp on a regular basis to avoid this because I have not seen this in their units. It is possible other manufacturers may add this feature over time.
It may be necessary to replace a lamp to eliminate this problem.