Trouble with RFID ID12

Hi!

I'm working in my arduino proyect with the ID12 module. When read the tags, I get ÿÿÿÿÿÿÿÿÿ or FFFFFFFF in Hex. I have read the link: RFID id-12 tagreader | Conversations with spaces where some comments say the something, but the solutions described are not valid for my case. I don`t what to do... =( ¿Any ideas?

Perhaps the module needs any net current stabilizer or any capacitor.... but I'm not sure...

Thank's so much in advance!