Celular Shield with SM5100B & Arduino Mega

Hello

One year and half has gone... on this time the GSM were keep it on the bottom of the drawer.
Last few days I've tried to put it back, but without success... I receive some trash by serial.

I'm using Arduino 0023 exe

It's strange, because the Arduino is the same, USB cable is the same, 12V power suplly is the same, SIM card is the same, laptop and OS are different... I'm using Windows 7.

Without power supply connected, I have reasonable correct information...

But with 12V power supply on Arduino, plus USB cable... Jesus!

I've tried with other Arduino Mega (2560 instead 1280)...
Changed 12V power supply...
Changed 220AC socket...
Changed USB cable...
Changed SIM card...
But the problems remains...

I read some about serial speed... that by default came with 115200, but on my case I doubt, because before worked well with 9600!

I don't know what to think!
I'm desperate...

Thanks on advance
Pedro Ferrer