When the mower starts surging after 10 minutes, hook the tester up and see if it will consistently jump a 6–8 mm gap. If it jumps strongly when cold but becomes weak or intermittent when hot, I'd be very suspicious of the ignition coil breaking down with heat. If the spark remains strong while the engine is surging, I'd go back to chasing a fuel-mixture problem.
You can get a spark tester under $20. Adjustable Ignition Spark Tester T&E Tools 3404
