You really should not be using "thinner" wire. It could overheat and melt the insulation and short out the connection if close to the chassis. It may be OK at low motor speeds but higher speeds draw more current. Also as these motors age and the armature and brushes wear they tend to also draw more current agrevating your problem. It's best to use the correct gage or if not possible use multiple wires of smaller gauge twisted together at the ends. Doesn't look good but will help.