Old ignition coils can be prone to breakdown or open circuit when they get hot. Had this problem a few times, once on my old 3YF, but particularly on British bikes and I've found Rover V8s are prone to it too.

Check the spark cold, and after it has cut out, on a hot engine and compare. Better still check the resistance on a cold and hot coil, or find another to substitute. In fact check all the HT side of the ignition while you're at it.

Also, another thing is I just remembered, I had an NX650 that showed similar issues, and that turned out to be the crankshaft pulse generator failing as the engine got hot. Heat can do funny things to electrics, especially cause increased resistance.

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