Here's some insight on the bulb life --
Heat, voltage, amperage, and resistance are all factors in the bulb's life.
1. Heat increases exponentially inside the reflector as the wattage is increased. The reflector is designed for a 50-65 watt bulb, but could handle about 80W without stressing the limitations, 110W is just too hot.
2. This brings us to another point, resistance. Resistance increases as the heat increases, thus altering the voltage/amperage requirements from the normalized draw outside the enclosure.
3. Voltage drops and amperage increases under these circumstances create stress on the filament, thus it fails pre-maturely.
These 100-110W bulbs are marked 'off-road use only' for a reason. They are designed to be used in a specific 'larger' (sometimes ventialted) housing, and are for short-term useage. Long-term use in a standard, sealed housing will produce the short life-span that you are experiencing.
I suggest you stick with the recommended wattage bulbs (50-65), or move up to the median wattage of 60-85. These mid-range bulbs are available from Hella and will last almost as long as the recommended bulbs. The output is much better than the 50-65W, but is not really noticeable as compared to the 100-110W bulbs.
I am not sure about where to get them, but I know somebody sells the 60-85W 'upgrade' Hella bulbs, I've bought them. Look around and you should find them. Price is around $10.00 each.
76 240D (W115) - 550K miles
78 300D (W123) - 200K+ miles
[This message has been edited by WmHarlow (edited 08-18-2000).]