What position are the lamp working at? horizontal or vertical?
I've watched the same problem 16 years ago on a "Metal-Mazda" MMF 160w blended mercury lamp because these was fitted horizontally into a particular downlight street fixture at my countryside home.
Mixed lamps are primarily intended for vertical up or down burning position, at which, the convective flux in filling gas allows a proper dissipation of heat generated by both arc tube and filament.
If a such lamp are horizontally or almost placed, the lower portion of filament surrounding the arc tube, overheats this, making to raise their pressure over the specified limits, giving as a result that the arc extinguises because the voltage between electrodes reaches the critical value to be unstable. So, the lamp suddenly turns off until temperature decreases and the mains voltage are enough to ionize the arc tube again.
However, these problem are not be seen on regular high pressure mercury (not blended) lamps too often, because they are intended for working in any position.
Sometimes the vicinity of the reflector over the lamp blocks the correct convective cooling of outer bulb at a so degree that the whole lamp severely overheats, allowing the arc could be dissapear repeatedly (in the best of the cases!).
When a such problem are seen, the most reasonable thing to do is to test the lamp at the same position, using the right ballast, but out of the luminaire to find out about the lamp alone or combined lamp/ballast/fixture "healt".
Another thing to test is the mains voltage. If this is too low, the arc tube could de-ionizate repeatedly when the pressure within it reaches the upper limit. This is a phenomenon I've observed too many times at my countryside home street lamps, which I've fitted with 80 and 125w clear mercury lamps, in a time when the line voltage used to drop at night, down below 190-200 volts instead the nominal 220-230 volts, due to a severe overload of the line.
Best regards.
M. Gonz?lez.
