My good lady's 650 shitehawk, sorry, nighthawk had the same thing, but only occasionally happened when turning the bars full lock to the right. After much cursing and trial and error I found a dodgy connection in the headlight was responsible. Why the coil wires were put in the headlight beggars belief, but as Steve said, it's an ignition related fault. Follow the wiring from the coils upwards, checking cleanliness and security of the connections, and looking for external damage to the wiring.