My 2004 S60's head and tail lights go out intermittently

it really could only be loose/faulty wiring. Take it to another dealer and don't use the first Volvo dealer again. To say a lighting problem can't be fixed is the height of ignorance.

It sounds like you need a new alternator. I had the same problem and the mechanic said that sometimes the alternator creates too much of a charge that results in blown lamps. I had my Alt. fixed (after replacing 5 bulbs in a yr) and had no problems. Hope this helps.