My son's quad did something similar. I was going nuts trying to figure out why it was erratically stalling and shutting off and was hard to restart.
One night while jacking with it, I got down to ensure that I was shutting the choke off all the way and saw where the spark pug wire was touching the cylinder head and it was arcing across the wire to the head. Short term, I zip tied the plug wire away from any metal but I really need to replace the plug wire. Since then, no issues.
Check that plug wire in dark, if you see arcing then that's your problem.
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