Hi, i have this problem and i do t know what can be i think i somthink about eletric but i dont know. There is new ignition coil and spark plug https://youtu.be/DVs6sJXzH4w
There still is an air leak. When your engine warms up the air leak may stop because the engine components expand.This means that you still need to find the leak and fix it. This could be because of old crank seals are failing because after you start the engine the crank seals or head gasket/base gaskets expand enough to seal.At cold start up, your engine is running lean because of the air leak and you may have a piston seizure.
Also sparkplug cable could be dropping sparks to the frame instead to the plug. Found that on mine incidentaly during night riding and seeing small flashes under tank...similar behaviour.