Really weird, but had a similar problem today...!
Bike equipped with a Pazon ignition, it suddenly dropped dead. Luckily I remembered 2 things; firstly, I changed from NGK #8 plugs to #6 yesterday, and secondly, I ran the bike about 1/2 mile at very low speed and left it idling for about 5 minutes.
When she stalled I tried to restart her, but she kept on backfiring, and refused to start. I left her to cool down for 10 minutes, and she started fine after that, but the problem reoccurred after about 2 miles at low speed. She never did this before on the #8 plugs so I guessed she was running a bit hot and thus pre-igniting. I had a set of spare #8 plugs with me, so I put them in, problem solved. Rest of the journey no more problem.
Maybe worthwhile to try a set of cooler plugs? Or retarding ignition a tad?