My A6 Avant quattro 3.0 has an intermittent fault my dealer cannot diagnose. If the car is in a position it may overheat, eg on a slow moving motorway roadworks etc, in a queue to get on a ferry after a long drive, and I switch off the engine to avoid overheating or the stop/start cuts the engine, it will not start again until the engine cools down, 10-15 minutes later. The engine turns over, but will not start.It once cut out at 70 mph without warning on a busy M6 after clearing slow moving roadworks. My dealer could only find a drop in fuel pressure from an unknown cause. It is more likely to happen when the outside ambient temperature is high, e.g. hot Summer day. My confidence in the car is low now. Any ideas?The temp gauge does not usually register a rise in engine temp and stays at midlevel on the gauge at 90degrees.