Hi, try using a lower wattage bulb. I think it is from the glass getting too hot. (maybe there are some kids in the neighborhood who like to throw rocks ;)

Here is a link that might be useful: Lake Lighting

I have had the same thing happen. they're plain old PAR38 outdoor halogen flood lamps (75 or 90W) and the glass lens just falls off. Filament still works, though. I suspect I got a bad batch at Home Depot. I've never had it happen before, so I don't think the wattage is too high. If it is, then it's a design flaw, since these are mounted outside in just the bulb holder.