New plugs, bike running fine, till it wasn't. Shut down again t'other day which makes about five times by my recollection. Again, it was when I throttled back approaching some downhill bends. Perhaps the wags will suggest don't close the throttle then? Re-started after maybe 8 or 9 kicks, ran home with no problems.
Today, investigated whether there might be a charging issue. Voltage across the battery, engine off 12.58. With engine running about 2-2½ thousand rpm that goes up 14.64 so I guess the charging ain't a problem.
Could there be some intermittent electrical problemo? And how to find it? People have suggested the coil breaking down when hot but I don't think it would restart after such a short time. Mind, I would try replacing the coil if I had a spare one. IIRC coil is BB dual output.
Next approach, when I have the time, remove the carbs, see if there's rust'n'grit, check and clean the main and pilot jets. Though I'm not sure iffy carbs can cause this sort of problem.