What information did you use to inform the choice of this motor?
Realistically you need to know the amount of power it will be required to produce.
converting the units 7.5 Kg.cm = 0.73549875 Nm
and using the max figures (no load speed and stall torque) that is about 1.5 Watts.
again using the max quoted figures
Locked-rotor current: 0.6 A * 6V = 3.6 watts
That is very little power for doing any real work.
So the motor is working very hard and likely the internal gears will be getting very worn.
Basically you need a more powerful and robust motor.