consistant misfire out of nowhere

I say carburetor myself .

My 70 duster that I bought last year had been sitting a long time and I replaced the tank.

The main jet in the carb clogged twice after first getting it on the road and has been fine since and it acted just like yours is doing.

Very simple fix take off the float bowl and unscrew the jet and clean out the hole , I was care full and was able to reuse the gasket both times. Blow out every thing in the bowl with carb cleaner and reassemble