Every time I see that happen it was because one of the brush holders was grounded. There MUST be an insulating washer on each side of the motor frame. So, you have the brush holder, insulating washer, motor frame, another insulating washer to fit in the countersunk hole, then the mounting screw. You need this on BOTH brush holders.
You can make an insulating washer from thin fiberboard, or even an old credit card. Something like the cardboard from a cereal box WON'T make a good insulating washer.