Electrics/ignition ?

I have just purchased BMW 328i 1995, well cared for 2 owner car.first chance to give a reasonable drive today and have had a most unusual fault arise, can anyone throw any light on it? Driven for 20 miles up to 50 mph normal roads, then entered motorway and after reaching approx 60 mph the dash lit up as though I had just turned on the ignition, and all equipment( speedo/fuel guage/ wipers etc) went dead, engine still running normal, before I could get to hard shoulder everything returned to normal. After a minute the same happened again for about 10 seconds and returned to normal did not happen again after 30 more miles ????
Any Ideas please....