Some questions regarding a servo motor's data sheet.

[selbstdual]

Currently I am using gear wheels to make the 180° servo rotate a shaft 360°.

This happens(The shaft is being stressed):

1. Servo gets the order to drive to a position X and does it.
2. Shaft rotates to a new position Y close to X.
3. Shaft successfully rotates back to X.
4. Shaft rotates to a new position Z being distant to X.
5. Shaft is to rotate back to X but stops at a different location - in between X and Z.

This occurs every time the shaft travels over an angle greater than 120°(60° without gear wheels).
According to my calculations the servo's torque is high enough.

Any Ideas of how to explain that?