I've recently had this with my 2000 L series, exactly the same problem. Bulb blew and I found it on the MOT pre-check so replaced it. A couple of weeks later it blew again. Replaced again. Okay at the moment, I think....
Suspect an intermittent connection that causes the bulb to flicker. This will increase the therma stress, the killer of all filament lamps.
Also had problems with the number plate lamp being intermittent. Wouldn't light when switched on but a quick 'thump' fixed it, until next time. This was due to the rivetted terminals not making a good connection to the PCB that the lamp is mounted on. Some work with a soldering iron soon sorted that one.