Hello My last C5 2001 2.0HDI estate Auto had the same problem, It first started at about 65000 miles and continued on and off until 80000 when it happened most days. When the dealer checked it out at first they found no fault, when they checked it at 80000 they said it needed a new box £2000.00 of the lease companies money. However after about a further 20000 miles the problem started again, I took the car back as the box was under warranty and they replaced and electronic unit on the side of the box and said Citroen had discovered a problem with this part and had changed the make! and that was probably all that was wrong with my old box. The part was approx 200.00