One thing to check is the inside of the fuel tank. If you have a pinhole in your line up near the neck, as the airplane flies the fuel moves to the rear of the tank. You now have the pinhole exposed to air and have an instant leaning of the engine. It took me quite a while to find the same problem. I changed the line inside the tank and no more problems. Another way to check for poor fuel draw is to hold the nose of the plane up for 20 to 30 seconds. If it runs OK with the nose up, than you can be sure that the problem is something else.