I experienced the same symptons, engine randomly cutting out, once just outside the M25 tunnel, and I too learnt that you could carry on by switching the ignition off and then on , without even stopping.
I eventually traced it to a poor pin connection in the 15 pin plug between the chassis loom and the engine loom, found by having the engine running and gently wiggling this plug in a certain direction, then you could hear the ignition relay trip out, so cuting the engine, then it would immediatley restart until another wiggle.
I couldnt imagine a cheaper looking 15 pin plug!
