I have noticed the same thing since day one, and I can't find anything wrong with the system. I have no add-ons (save my satelite radio, and I don't see how that would affect anything), but I do have the towing light wiring package, along with the tow bar. Again, I don't think that would do anything, as it is all Subaru parts.
I recently replaced the factory battery, cleaned all connections and I am still seeing the drop off of the lights. It may be normal, but the 2003 Outback I had never did it, and my Tribeca doesn't do it either.
Anyone have any answers?
Keep it under 90, folks!