I had this exact same problem with my 1977 MGB for a while... I figured out
that if I wiggled the loom of wires under the hood on the passenger side of
the fire wall, the ignition would come back on and I could start and drive
until the next time it would happen. I finally broke down and sourced it to
the round ignition relay just above the coil. The contacts inside were
loose-I could move the spade connectors back and forth easily. Once I
replaced it, the problem went away for good.