Not Subaru related, but I've noticed that there is an issue with Chevrolet/GMC trucks and their daytime running lights doing the same thing. I asked a co-worker of mine (who has a GMC truck) and he mentioned that there was actually a recall on the truck lights. Perhaps there is something similar with these model Subarus?
Other than that, the only other thing I can offer for advice is when you replace the bulb again, nail it with some lithium grease to prevent moisture. Sounds like they might be shorting out prematurely?