I had this issue constantly when cutting circles. Bit would grab and gantry would run wild. I figured out my problem was my stepper drivers. I had adjusted them according to sound and heat from the motors. This pretty much gave me 4 separate stepper motors with different torque. I decided to buy a multimeter and adjusted each stepper driver to exactly 0.6 amps. This actually fixed the problem completely. My machine cuts solid now. May be worth a shot to check your stepper driver potentiometers.