Assuming you have checked for obvious mechanical problems and found none, the fact that it shifted when it transitioned from printing the outline, which uses both motors, to the infill, which uses a single motor, suggests that you're trying to print too fast for your motors and/or have the acceleration and/or jerk set too high. You might be able to get a little more torque out of the motors by tweaking the current a little and/or raising the power supply voltage, but I suspect you're going to have to adjust your expectations about print speed and start cutting back acceleration and jerk.
Ultra MegaMax Dominator 3D printer: [
drmrehorst.blogspot.com]