Thanks for your answer.
As I said, sometimes the motor will be "locked", which means,I can barely move the motor when it's powered up (i need to use force). and sometimes I can move it, but it just wiggles around.
I use a external 9V Power supply, which is also being grounded to the arduino (9V Runs from the PSU to the both center tap wires and the ULN2003A, and Ground is on the ground rail, and also connected to the arduino).
It works with the 5V coming from the arduino itself, but the motor is very weak, but has the same problems as with 9V.
Edit: I don't understand what you want me to do with the link you posted? :o