It might be the drag, click the button that has a triangle with arrows pointing to it, and see if there is more red on one side than the other. If there is go to the XML menu and set calculateDrag to false. That should fix it. If not then it may have more weight on one side, if so just mirror the aircraft.

@tsampoy so how do I stop this

Check your CoM, if it's in the center. Maybe the engines don't have equal thrust or are parallel to each other (if you have engines on either side)