Joel is correct on his assessment, the carbon brushes sticking could very well be a issue. However, I would also try reseating the carbon brushes and see if that helps. This could very well help since the problem started after sanding the armature.
If it is indeed the commutator going, there is a seller on Ebay, mickeymackmark1, who has the commutators for the early and late 1205 motors. So at least you could get one and take it into a Aerus dealer to have your motor fixed and get new carbon brushes installed too.
Rob
PS - For anyone reading this, DO NOT SAND YOUR MOTOR COMMUTATOR! Use a armature stick instead from your local vac shop. If you sand the commutator, it can ruin it by sandpaper grit getting stuck in the grooves. A armature stick will not only clean up the commutator, it will help the carbon brushes seat correctly and is much safer.