Heat build up is the problem. One common problem along that line is true of head lamb bulbs of all wattages and wiring: Always be sure to NEVER touch the bulb with your bare skin.
The least little bit of oil (fingerprint) will darken from the heat, absorb and concentrate heat at that point, interfere with heat dissapation, and contribute to if not cause early bulb failure. Thats why the directions with the bulbs warn against handling 'em with bare hands.