These units had one crucial problem which is that the drive gear get hairline splits in them after substantial use due to the pressure that was used to press them on the motor shaft from factory.This causes same problem and there is no quick fix as you have to pull it down to get at the gear and remove it of the motor shaft. At the same time (be-careful) of twin adjacent gears as they are loaded with a shockabsorber spring which tends to fly out if disturbed.
Inspect the gear for splitting using a magnifying glass as it is so small, if so, the smallest amount of superglue to the inside of shafthole and gently squeeze togeather with small pointy nose pliers.
Re-check for superglue overrun between gear teeth, a little bit of fine wet and dry sandpaper fixes that.
Also ream out a small amount of the center hole so it is a sliding fit on motor shaft and the end of a pin dipped in superglue to bond it back on.
Re-assemble and test.
Frank
March 2007