Jawa 210 (1983 model I think). Just got the carb cleaned up and tuned right (14.12 SHA clone). Idles great (if a little high), starts with one kick. After riding around at about 30 mph for a few minutes, the engine rpms will suddenly start to drop until the engine dies. Opening throttle has no effect. I'm usually able to restart the bike without much problem - the note here is that I almost always check the spark plug cap to make sure it's tight. It generally feels a little wobbly. But I'm not sure this is the issue because sometimes the bike re-starts without me poking at the cap.
Before I go throw any more money at it, I'm going to try a fresh spark plug. Beyond that, I'm thinking cap/boot/wire replacement? Is something in the ignition overheating? Thyristor, coil, .... ? It seems like it wouldn't idle well if it were a spark issue, but I'm out of ideas.