Did you replace the glow plug in an attempt to fix this problem? OR Has this problem just started since you replaced the plug?

Sounds too lean to me. or a tank problem, tune the motor to peak rpm then richen it a couple hundred rpm then recheck holding the plane with the nose up 45 deg then strait up to simulate a take off.

Tank vent plugged? Muffler pressure nipple plugged? Hole in any of the fuel/Pressure lines?