Skid Steering issue

Hey all. I’m trying to tune skid steering on ardurover 3.3.0, pixhawk 2.4.8. It’s almost done and my vehicle with 6 wheels, 3 on each side, can move, but not correct. If I’m trying to ride forward/backward or turn left/right, everything is good, but in case riding forward and left simultaneous, throttle suddenly changes from 100 to -100, I mean vehicle is trying rides backward and turning left. What should I do to solve this problem?
I don’t know why, but in case riding forward and right everything is good. It works right. And, yeah, if I’m trying to ride backward and left, it switches throttle to max(goes forward).
I’m using QGroundControl(and Mission Planner)
Turnigy 9x with ppm-encoder
Pixhawk 2.4.8
Ardurover v.3.3.0~dev
p.s. i’m using mix only for 3-pos switch(for Hold, manual and auto modes), on “display” tab(in RC’s menu) everything is correct)