When lamps are blowing regularly the main suspects are:
1) Poor quality lamps.
2) Over-voltage, +5% over-voltage will more than halve the life expectancy of the lamp.
3) Loose connections.
4) Poor quality lamp holders, particularly batten fittings.
5) Power quality, surges, noise and harmonics can all shorten the life expectancy of the lamp.
[FONT=&]6) Rapid switching of the lamp ON and OFF ie. Short duration use.
Specifically to the OP check that Dichroic lamps have not been fitted, dichroic reflectors reflect visible light forward while allowing the invisible infrared light (radiated heat) to pass out of the rear of the fixture, resulting in a beam of light that is "cooler"[/FONT][FONT=&], but if the radiated heat is not sufficiently dissipated this leads to early lamp failure, and in extreme cases burnt fittings, wiring and/or surrounding building materials.
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