It's been pretty well covered above, but the short will be a breakdown of insulation somewhere. The king lead is an obvious candidate as it's taking 8 pulses for a single pulse on each plug lead. A wide plug gap can force a miss, but you've already changed the plugs so they should be OK. There was a workshop note for MGBV8's which advised not having two specific plug leads next to each other on the offside bank. I don't remember now which these were - but it was because the induction from one could upset the other if they were side by side. Might have been 4 & 6 because only 3 fires between them, or 4 & 8. (1,8,4,3,6,5,7,2).
Follow Ray's advice and look in the dark.
