I´ve heard about this problem before. It´s the ignition pickups. They are coils that generate a pulse when a small magnet passes them as the crank rotates. They do the same job as contact breakers. You can find them behind the right-hand crank cover.
The reason they work OK when the bike is cold is because this type of failure is due to a field breakdown with a build up of heat. They are not an expensive part to buy and are very simple to fit. I´m 95% confident that changing them will cure the problem for you. 
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