UKHeliBob:
Are you sure that you have the sensors aligned properly with the orientation of the quad ?If not then when the software attempts to keep the quad level it will increase the speed of the wrong prop, that tips the quad further out of level, the prop speed increases in an attempts to correct it, the quad tips further...
I am sure that you can see that this does not end well.
yeah actually the MPU6050 is on the place. well it slightly offset due upper f450 frame has a hole. Also I made sure that with calibration. gives me desired ypr except +/-1 to +/-3 deviation in pitch and roll.