My 2003 Jaguar XJ Super 8 automatic has recently been making an occasional, mild, gearbox "thump" when changing up from first to second gear, together with an amber engine warning light and a gearbox fault display that self-cancels. After the third attempt at diagnosis the dealer recommended a replacement gearbox and throttle body with a bill north of £4000. A mechanic friend suggests the gearbox may be getting misinformation from the ECU, since the problem only occurs when crawling in traffic queues or similar situations. Needless to say, my confidence in the dealer's analysis is undermined and, being a regular reader of your excellent column, I am desperately hoping for some much needed guidance.