The comm is probably in bad shape, if it's not more serious like open windings or shorted lands. I'm not an expert but on drills I have sanded the commutator. Inspect it and I think you'll find it is damaged (excessive sparking causes damage). You have to be careful not to damage the wires attached to the segments. I've used a woman's emory board because it's fine and flat. Particles removed can go between the landings and could be conductive. A toothbrush and/or air might help. Hold the emory against it and turn it. Routers run way more quickly than drills etc, not sure if this might be too crude of a fix for them but it worked several times for me on slower motors. You only need to clean where the brushes ride.