If it has always done it, could you have the wrong wattage lamp for the
fixture? I think the ballast has to be matched to the bulb, just as with
flourescent lamps.


Yes it does, mercury lamps are quite tolerant of over and under wattage
operation compared to other discharge lamps, but in either case lamp life
suffers, sometimes dramatically. If the cathodes are too hot they evaporate
and blacken the arc tube, if they're too cold they sputter and blacken the
tube, either way the lamp lumens depreciate substantially over time while
power consumption remains the same.