I think I finally sussed out what was happening. The first time it happened the battery was almost dud. the next time with the new battery it was down to a bad positive connection, it had come partly loose and I only discovered this when I went to start the car one day and there were no lights on the dash. Since I recharged the battery and made sure the positive connection was perfect it has never happened again. The positive battery on this car is a bad design as the two right angles above the clamp holes are facing the same way which means that one side of the clamp can climb on the other before it gets tight on the post. I hope this makes sense to anyone who reads it.