Randomly motor problems with FC APM 2.6

Goodmorning everyone,
I would like to expose my problem to which I can not find a solution.
First of all, I list the characteristics of my Copter:
Frame DJI F450
4 FullPower 2814M 780KV motors
4 Esc FullPower
FrSky X8R receiver
Gimball Tarot 3D
Turnigy 5200MAh 3S Multistar battery
Propellers 12 x 4.5
Flight Controller Arducopter 2.6 with firmware 3.2.1
Beitian 8ch GPS external
Sik Radio Telemetry
Trasmitter FrSky Taranis 9D plus
On the PC I installed Mission Planner 1.3.56 built 1.3.6672.30243
Ok … my problem is this:
After doing all the calibration procedures of the FC (Radio, Compass, Magnetic interference of the motors, Accelerometer, Calibration of the ESC, not in this order of course!) And after having hooked the GPS signal, I prepare to flight in Stabilized mode. I proceed to arm the Copter, with the centered controls. (I chose to have the propellers that turn at idle when the copter is ready on the fly)
I begin to accelerate and it is very evident that one of the engines runs at a lower speed. It can be the right rear or the left front indifferently. At this point I move the commands of pitch, yaw or roll keeping the gas at the same level and the engines respond perfectly, when I release the controls, the engines all run at the same speed or the one that runs slowly has changed randomly.
This effectively prevents me from taking off because the Copter rolls on one side or rears up without being stabilized. I have repeated all the calibration procedures several times and are all successful. I thought about a problem with the radio, but I use it with another F450 frame and another Frsky X8R receiver, but with a KK 2.1.5 FC and I have never encountered this problem.
I tried to reinstall the firmware in the Arducopter, but to no avail. It seems to me to be a problem of the FC’s response to the receiver’s commands, or a bad ESC command from the FC … but I can not determine the solution.
I hope I managed to explain my problem well and I apologize for my bad English.
Can anyone help me?
I thank those who answer me in advance.



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