Here's some ideas...
First, you need to get away from the bike for a while. I know what it's like to have one piss you off, but you'll never get anywhere with it until you clear your head.
Next thing, remind yourself that the engine needs only four basic things to run: Air, fuel, compression and ignition. It can't mess with you and not run if it's got all four things, it's just a machine and you're its God.
Anyway, enough of the pep talk. From what you're saying, it really sounds like something's funky with your ignition.
I don't know jack about Mobys, but here's some ideas generic to points style ignitions:
Check it over with a fine tooth comb, make sure all the wiring and connections are clean and tight, and check your points with a meter (you'll have to disconnect them first). They need to be zero ohms when closed, anything other than that and your spark will be erratic. They can look good to the eye and still be bad.
Dirt and grime are bad anywhere in the ignition. It needs to be clean. This goes for the underside of the flywheel, too.
Ff your Moby has one of those CEV "run/stop" switches, check that. They're cheezy, and if the cover is one just a little crooked, they'll short and kill the engine, sometimes intermittently.
Oh, and I agree with Dan, get a new plug. That's a lot of miles, especially if the bike's not been running well.
I really hope that this helps, even if just for moral support. Good luck!