oxidation of the filament :cheesygribwall said:
if i remember my high school electronics correctly (there's a good chance i don't, it was first period) then current (amps) increases in relation to an increase in impedance (resistance). if that's the case a poor connection at the fixture, a faulty lamp socket, etc. might cause the current through the bulb to be higher then it's otherwise supposed to be and that might be causing the premature failures. just a guess.
