Many variables can cause the problem the problem you have: WOT set too lean; fuel tank position too high; wrong heat range glow plug; insufficient muffler pressure to fuel tank.

Set WOT with a slow pinch test on the fuel line. If set correctly, you should hear a slight increase in RPM's. If set too rich, you will hear a larger RPM increase. If set too lean, you will get a drop in RPM or the engine will die.

Tank position: the centerline of the tank should between in line with the spraybar to 10mm below it. Your positioning of your needle valve should not have an effect. Commonly, people run into the problem of a too high tank if they mount their motor sideways or inverted on an airframe designed for an upright engine and not reposition the tank.

As for the glow plug, you need a hot heat range plug like the OS #8. A colder plug can give you "flame outs" and not show it on the ground.

Check the pressure nipple on the muffler for plugging or constriction and your tubing for a leak. Well used mufflers will get a buildup of varnish or gunk. Remove the nipple, clean off the face and run a drill or wire through it to clean out the hole.

Other than that, my only other recommendation is change your propeller to a 10x5 or 10x6. An 11x6 is too much load for that motor.