Lamp flicker sometimes means the arc is just moving from one point to another on the anode/cathode, and putting it on high lamp can alleviate it. You may find that after the arc has bedded in again you can go back to low mode without the flicker.
UHP lamps are arc lamps rather than filament lamps like we have in our light bulbs at home, so the current has to arc across two electrodes to produce light rather than heat up a filament. Usually the point of least resistance means there is one arc point on each electrode, but sometimes as the arc point wears, another area on the electrode can offer similar resistance so the arc jumps across to there, and then back again, so you get flicker. Putting the lamp on high allows more current to flow and that can make one arc point more conductive (whichever offers least resistance to conductivity) so the lamp flicker goes. As the arc burns in that electrode, the new arc point becomes the point of least resistance so going back to low means the same arc points remain in use.
Hope that makes sense