I will try tonight! In any case, it works correctly for a few hours, getting a constant stream of data without any error. Then he begins to work in this bad way.
Another problem: when I try to reset by pressing the button on the shield, in the new reboot, I do not receive any data. Only if I remove the external power supply (Vin, 9V), it starts to work properly. Do not use USB power, only external power supply.
I use the Arduino Ethernet shield + Official + SparkFun USB host shield, connected in this way: http://arduino.cc/forum/index.php/topic,51874.0.html