Debri in pump? Leak in pump gasket or loose screws on housing? I don't check fuel lines, replace on a regular maintenance schedule. With a carb flush, I blow all out with compressed air. Check crank for play. Any play negates valve adjustment. All bolts on head and mill tight? Or any other screws for that matter.

Here is another one that I have read of and experienced myself. Also it took quite some time to resolve it. With the increased fuel demand in the summer months, some fuel manufacturers have been getting sloppy and small particles, as well as shavings of plastic have been showing up in fuel jugs. Its the same plastic of which the jug is made. Symptoms as you have described, yet no way of finding the cause of the engine fatigue. Finally found the problem, when filling, the fuel line would blow off from pressure built up due to the restriction. The particals and shaveings were everywhere. In the tank, regulator, tees, lines, needle passages. Now I transfer jugs and screen the fuel as it pours to another jug. Filtered clunk in fuel jug, filter on fill line, filter on fuel line in plane and finally filtered clunk in fuel tank on plane. Good luck. Joe