DIAGNOSIS Excessive vibration means an imbalance. The blade is either loose, damaged, or incorrectly installed.
FIX Stop using it immediately. Remove the blade. Inspect the blade's mounting hole for cracks or oval wear. Install a new, undamaged blade. Ensure it sits flush against the tool's mount before tightening the bolt.
IF THAT DIDN'T WORK The eccentric bearing inside the gear housing could be failing. This requires professional repair. Listen for a grinding or knocking sound from the body with the blade removed and the tool running.
HEADS UP Running a badly vibrating tool accelerates wear on the internal mechanism and is a safety hazard. The blade can fracture and eject.