Are you running 55w bulbs? Guz has a good point, are you sure it's inside the enclosure? It could just be oxidation.

Regardless the halogens are worth it in my opinion, but that's because hid's without projector lenses have horrible spread and look tacky/ricer. Retrofit or go for some Phillips HIR2 bulbs, I had great luck with those before I went retro. A bit brighter than silverstars and I didn't have to replace them every few months, 1 and a half years on he same bulbs.