The symptoms that you are describing are usually caused by the brushes not making good contact with the armature. this can be caused by several things. It can often be solved by simply running the motor in reverse for a few minutes. It is not a bad idea to do that every once in a while. If that does not fix it then remove and inspecting the brushes for any damage, clean the brush holder and the armature with a little compressed air, then rotate the brushes 180 degrees and re-install them. It appears to me like the top brush is slightly more worn than the other that is not unusual. it is caused by just having a little more spring pressure on that brush. The caps should be adjusted som the brushes make contact with the armature but too much pressure will cause the brushes to wear more quickly and create more carbon dust. If that does not solve the situation there is also a few other things that can be the problem. The power supply/ speed control unit or possibly the motor windings. but that would not be too likely.
Again looking at the wear on the top brush and the spring, I would consider replacing brushes. ( message or email me)