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