I just repaired one of those and I had the same problem. I noticed that the armature shaft had too much end play allowing it to move slightly back and forth in the direction of the shaft. When the motor was running at low speed the shaft bounced up and down thus altering the gap in the speed sensing unit (or whatever it is called). This movement sent erratic information to the speed sensor causing the "lumpy" running. At higher speeds the shaft didn't seem to bounce as much so the problem went away.
The fix is simple. You don't even have to take off the motor, just the cover over the electronics. Slacken the lock screw holding the fan assembly in the motor shaft under the circuit board (you don't need to remove the board either). Then remove the slack by pressing the fan inwards and tighten the lock screw. Hold the shaft back with your hand where the shaft protrudes through the top of the machine. You might not want to put it too tight to stress the bearings.
In my case this simple adjustment made the motor run much smoother. It ran just a bit rough only at the lowest speed. I adjusted the minimum speed (with the screws that hold the circuit board) a little higher and now it runs quite smoothly at all speeds. I guess new bearings might have been needed too. I changed the brushes, though.
Tim, March 2007
