This POP - could it be described as a bang that sounds like its coming out of the airbox???
If so I had a similar problem which I traced back to the D.I.S. unit, it would run perfectly then bang a puff of smoke and the car wouldn't start, i'd disconnect the DIS and connect a spare one, turn the car over a couple of times then reconnect the proper DIS and the car worked fine, i never did work out why???
That cars gone now (RIP YDF700Y 12.8.1982 to 16.7.2003) and the problem went with it but i do still have the DIS if you're local and want to give it a try. (i'm just inside M25 nr dartford)