[sonicj]

try cleaning the f&r before replacing. denatured alcohol & a toothbrush, paper towels and compressed help as well. use a protective lube like boeshield, corrosion x, or preservIT.

imo, sounds brush related. you can remove the motor endcap to inspect, clean and/or replace the brushes. unevenly worn commutators can be serviced by your local motor repair shop. (or anyone with a suitable lathe)
-sj