1. If the Boring Bar is adjustable, be sure the lock screw on the adjustment mechanism is tight.
2. Check the cutting tool to be sure it is sharp and not damaged.
3. Be sure the correct Tool Offset values have been entered and are currently active.
4. Check the Boring Bar to be sure it is not bent.
5. Be sure the Tool Holder is properly aligned with the centerline of the spindle.
6. Make sure the Depth-of-Cut or Feedrate is not too high causing the Boring Bar to deflect.