Very difficult to say, but I would probably go with a loose electrical connection, bad charging system or reg/rec, or dead/dying battery.

What I would do is go out for a ride (in circles so you're never too far away from home!) and wait for it to cut out again. Carry a nice long flat blade screwdriver with you, and as soon as it cuts out whip the clutch in and turn the fuel tap to off. Loosen the drain screws on the float bowls and see if there's any fuel in there! If there is, then it's doubtful it's fuel startvation, and you've ruled it out of the equation.

There's definitely no problem with running the lights with a fully operational charging system. When I used to have lights I would run with them on all the time for safety/visibility. If the charging system or battery is not tip-top though it can cause all sorts of problems with the ignition system. The battery on my MC21 had the nerve to fail recently after a good 10 years of service, and it failed over literally the course of 10 minutes! It was fine when I went out. I did a rather spirited run for about 15 miles out and back, and then a plug chop, and all was hunky dorey. By the time I had ridden along the beach and back home (only about 5 miles total) it had breathed it last breath and was unrecoverable!

Check the battery condition and the regulator/rectifier output. Remember even most "sealed" batteries can be opened and topped up, and keeping an eye on the levels will significantly increase its' lifespan!
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Andy.
NSR-WORLD.COM

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