Like NMK suggested start with the basics.......spark plugs

I wonder @ times the quality of NGK plugs, always chalked it up to the power sports industry as I've seen a multitude of NGK failures in watercraft & off road products. Then an automotive tuning friend of mine mentioned problems he has had with NGK plugs out of the box

The problem magnifies if the unit is started & shutoff quickly such as running it in & out the shop.
I will bet your problem is that simple.