Ive got the same problem and have not been able to find out whats wrong
Ive had my C5 for 9 years my headlight bulbs blow approx 3/5 times a year...its so annoying because they are a pain to fix never mind the cost. Garage cant help me..they just say buy the expensive bulbs (which ive tried and no difference what so ever)>
Duncan Gould
May 2011