Evening , been reading your postings & trawled my fading memory . From my findings , i agree the brushes required changing , BUt you also need to clean the commutator . I use garryflex blocks & wash off the comm with brake cleaner as it leaves no residue , use the finest grit block gently , it will remove any dark marks & wear ridges on the commutator
http://www.axminster.co.uk/garryflex...cleaning-block
I would also wash out the brush holders & springs as these can become very dirty and stop brushes from pressing on the comm properly and even bounce off when the comm spins at high speed .