I'd be taking the brushes out to make sure they have a decent length. They should be long enough to have a little spring pressure when fitted.
Have you bench tested the motor to see if it's sparking in operation? Or looked at the visual condition of the commutator?
Also, your X1 is 850w IIRC. For appliances above 700w, a 13a fuse should be fitted. Nobody really uses 5a anymore.
Check the length of the brushes, make sure the comm is clean and the motor not unduly sparking in use, if all OK, fit a 13a fuse and see what happens then. We'll go from there.