Have you checked / replaced the yellow connector in the steering column? If you open up the 2 halves (one yellow, one black) its frequently melted inside and suffers a poor connection. It supplies power to most circuits on the car.
Thanks for the pointer, yellow connector was actualy ok but one of the connections on the back of the ignition switch was loose. Must have been like that for some time as now its re-soldered not only has the cutting out stopped but the car is running a lot smoother.