when in a stall you need more power not reduced power......sounds like a couple of problems......only 50% power on takeoff, always takeoff and go around with full power...better to have the power and not need it than to need the power and not have it.
Elevator trim as mentioned above....ensure your elevators are centered when the elevator trim knob is centered and the stick is at neutral.
CG.... is measured up from the leading edge of the wing then aft 60 mm not back along the the fuselage from the leading edge 60 mm.....if you measured back along the fuselage 60 mm, then you are tail heavy.....
Lastly thrust angle of the motor.......if your motor is pointing up it could cause you to lift off early...most motors are set slightly down .....the thrust angle should be set in by the manufactuer.....or called out in the instructions.....
hope you find the problem.....remember power on takeoff then reduce power once a positive rate of climb is established......good luck