consistant misfire out of nowhere
Before ANY of the above do a simple voltage check at the coil pos with the ignition in the run position.
If you have a ballast resistor ignition it should be about 8-9 volts.
(You did say it changed with a different coil) :-)
It happened suddenly, so I wouldn't suspect a lack of fuel right away. (would probably be popping and stuff anyway)
It was running fine till you tried to leave the stop (timing jumps usually happen after a shutdown)
I say it's a weak spark issue.
While it's running you can jumper right from the battery positive to the coil pos temporarily, and if it acts good again, then you know you have a low spark/voltage problem.
If it doesn't change you can move on to harder things.
I always like to start with the simple and easy stuff first.