Engine bogs when using dipper

Hello guys , I am slowly getting the bugs worked out of this 4300c linkbelt.i do have a problem when I use the dipper in function I get a bog in the motor as if the pump doesn't go into its high pressure mode and stays in a high volume mode.i have replaced fuel filter cleaned screen at lift pump ,and don't seem to have the problem during other functions. If I pull back hard on joystick for dipper in it doesn't seem to bog but if i try to feather it for fine grading it bogs and almost kills...

Engine bogs when using dipper

Engine bogs when using dipper