A4 2.6 Avant Engine dies

I've got a 1997 A4 2.6 Avant  (petrol) with 52k on the clock. It's been dealer maintained since I got it 5 years ago. Now I seem to have a major problem – starting from cold and after 2 to 5 minutes driving
the engine just cuts out with complete loss of power. Then I can't start the car again without constant turning the ignition for about 10-15 minutes. This has happened several times now and in increasingly dangerous places – i.e. at traffic lights, major junctions.  I placed the car with the VW/Audi dealer and there was no fault shown on the diagnostics. I'm putting the car in again with the dealer this week but they say that if it doesn't show up on the tests there's not much they can do until I ‘demonstrate' it happening. I just think the car is very unsafe to drive especially around busy a City. Can anybody offer me advice (I'm not technical)  as I'm getting pretty desperate. Thanks, Nick
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Fuel starvation? Pump, crap in the pipes, etc?
Does it turn over but just not fire? Are you getting spark in the plugs?

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dies. Cleaned the Coils as the AA (UK) said that may be the problem but still does it. Anyone else with A series with larger engine had this problem:? BTW it's an automatic.
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