Ok, so it's not the gas cap. I tried to cut the lawn today and sure enough, 15 mins into it, it started to sputter. I immediately open the gas cap and it made no difference. It still quit. I got my inline spark tester and I got sparks. But maybe by the time I got my tester from my garage and walk back to my mower, it must have been about 5 mins. Maybe enough time for spark coil to cool down??
I'm going to order 2 new spark coils, spark plugs, a fuel filter, and a fuel pump. Easy enough to change the pump and filter so if those are the culprits, I'll return the spark coils. Amazon Prime cost me nothing to return them anyway.