Please help mechanics, 83 cb650sc losing a coil randomly?

I'd say you have two issues here. The reason it cuts out is because of a bad pulse generator (as Honda calls their pickup coil). Intermittent running is common with that problem, at least initially and often, but not always. When intermittent running is the symptom, everything is fine when things are cold, but it quits firing when things get hot. Eventually it quits completely. Other times it runs fine when you shut it off and when you restart it, it never fires again. With the cover off of the pickups, the spark plug laying on the engine block and the ignition turned on but the engine not running, you should be able to make the plug spark by momentarily touching the metal center of the pickup with ferrous metal like a screwdriver. Every time you tap it, it will fire the plug.

I highly doubt it's the ignition unit. They have a single primary input circuit, but two switching circuits, one for each coil. For it to be the problem, it would have to be a switching circuit and the most likely thing to go wrong is the switching transistor. They normally work or they don't and working intermittently is highly unlikely.

Your secondary problem is old plug wires that leak voltage when they get wet. If the plug wires are molded into the coil, there's no replacing them. The best thing to do is to slip a piece of rubber hose pver the plug wire to provide added insulation.

I think you are on the right track.

If you have a maintenance manual it should give a resistance tolerance for the pick up coil.

I suggest checking the resistance of all the 2/3 ignition system wiring while shaking them. I had a broken wire in the ignition circuit of my 79 kz1000 that caused a intermittant problem and it took a while to find but in the end was a cheap fix for a comparable problem.

Check all grounds. Take the tank off, fire up the bike, then mist water over the coils to find arcing.