Ah, let me jump in with my old favorite....check your coil connections! Make sure it is very clean, no sign of arcing, and a good tight connection. Mine was not, and it did the popping thing, fouled a plug, went cold, and quite generally drove me mad until I figured it out. I still haven't recovered completely, some would say, even though the bike has. If the connection is iffy, just change the female connectors. Good luck.
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-Jim
'93 MC21