Next time t happens, quickly open one of the float bowl drains, and see if there's fuel in ther - it should be a strong flow from a full carb. If not, you're looking at a fuel starvation problem. If there is fuel, then I'd be looking at an electrical issue, as things like coils can break down when they get hot.

Have a look at the connectors on the coils, to make sure they're not blackened or pitted...
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