GPRS Shield + servo => vibrating servo

Hello,

I'm using a GPRS Shield (Seeedstudio sim900) to send and to receive data from my server. Everything works fine.
However, when I'm connecting a Servo on a pin (pin 5 for exemple) the servo doesn't work correctly and vibrates.

If I'm using the same code without the GPRS Shield, servo is OK.

Does the GPRS shield interact with PIN 5 ? (The seeedstudio documentation describes the pin 5 as available).

Do you thing that it could be a power supply problem ?

Thanks a lot for your help !!

Do you thing that it could be a power supply problem ?

Possibly. How is the servo powered? Didn't you think it important enough to mention that?

Servo is powered by the Arduino and the Arduino is powered by USB...
I have ordered a 9V/1A power supply to be sure.

Thanks for your answer !

Hi, sorry for my english i'm french..
But i have a sim900 (seedstudio) too and i would like to command a servomoteur with my phone(sms).
but i don't find program for this.. must i use Arduino as interface or a other software?
have you got the programme that i need ?
Thanks a lot for your help