A vacuum leak in the bulb will cause this oscillating behavior. Basically, the air that is leaking into the bulb is quenching the ionized gases. As mentioned above, getting a new bulb should clear this problem up.

A HPS bulb can operate in either a horizontal or verticle position. It's the metal halide bulbs where one needs to be aware of the orientation.

__________________
Talk, without the empiricism to back up your statements, is dogmatic BS.