The OEM xenon headlights on my B7 A4 have started flickering intermittently. Doesn't seem to follow a pattern, they'll be fine for weeks then start and clear up within the same journey.

Is it a common fault with the balasts or is there another (Cheaper) underlying cause?

 Half of the time plugging them in wont bring up faults,lots of people have just changed both bulbs even if one is flickering(because of the voltage loop one maybe taking more power than the other) even though its not a cheap option try those 1st you can get genuines off e-bay for about £35 each,if not it probably the ballast units which i should imagine are £200 upwards per side