Assuming you bought it at your local shop, take it back there, remind them it’s under warranty and insist they fix it. Tell them exactly how to replicate the issue, take video, take a few logs to their shop and demonstrate, whatever. If the issue is as you described it should be fixed under warranty. If they send you away no trouble found, take it home, replicate the problem and show back up tomorrow with another video. Do that a few times and they are going to have to take you seriously.
IMO, the more you mess with it, the more grounds they have to deny a warranty claim and the less likely you will ever get it fixed.
At the same time, you insist it is a spark issue. Why? From my armchair it sounds more like a fuel vent, fuel line, or fuel (vapour lock) issue. When it stops, pump the bulb a few times. Does it stay full or empty immediately? I don’t see where you mentioned fuel. What are you using, when was it bought?