My 2004 A4 Avant TDi Auto (6Speed-Tiptronic) has started displaying weird problem. Every so often it over revs and will not accelerate as if it has gone into neutral. Sometime the management fault light comes on the dash but not always. After a few seconds everything returns to normal although if light is on it remains on. It has been to Audi specialist who say diagnosis say main ECU has lost communitcation with Transmission ECU and I can replace Transmission ECU for about £600 which may not fix fault. If it doesn't then the next step is to replace gearbox! (about £1600). Car has done 61K miles. Is it worth replacing or is this the start of something major?