DIAGNOSIS Worn motor brushes or a failing motor are the most likely cause. Reduced power is the symptom.
FIX For the 2953, the motor is a sealed unit. You cannot replace just the brushes. The entire motor assembly must be replaced.
IF THAT DIDN'T WORK Check the battery contacts on the tool and battery. Corrosion or dirt causes voltage drop. Clean with a pencil eraser.
HEADS UP Don't confuse this with a bad battery. Test with a known-good, high-output M18 HO battery first.