Hmm, I've had three different bulbs in mine, the oem's in twice and in all those times haven't had a problem. I not settled on the silverstar bulbs and they've been there several months. Do you ever stall the car when it won't auto stop? I remember this being the reason for one person because if you stall it it kills the DC/DC and when it restarts the inrush surge in voltage will shorten bulb life. Also do you start the car before turning the head lights on? Turning them on before starting the car would create a similar situation.
Other guess would you possibly have any moisture in the headlight housing for some reason? My only other guess is your DC/DC might be working out of the spec'd voltage. Do you get any fluctuations in the intensity of the light?