Arduino NANO 3.0 issue

Hello to all,

I've two Arduino NANO 3.0.
One of them is not more recognised after some hours without problems.

On dmesg I obtain:

Jan 18 22:54:34 x3fi-book kernel: [ 3144.056064] usb 2-2: new full speed USB device using uhci_hcd and address 77
Jan 18 22:54:45 x3fi-book kernel: [ 3154.720070] usb 2-2: new full speed USB device using uhci_hcd and address 88
Jan 18 22:55:17 x3fi-book kernel: [ 3186.312085] usb 2-2: new full speed USB device using uhci_hcd and address 102
Jan 18 22:55:26 x3fi-book kernel: [ 3195.144072] usb 2-2: new full speed USB device using uhci_hcd and address 108
(...)

I'm using Ubuntu 10.10.

One time it was recognised for not more than one second and then disconnected again:

Jan 18 22:54:30 x3fi-book kernel: [ 3139.592081] usb 2-2: new full speed USB device using uhci_hcd and address 70
Jan 18 22:54:30 x3fi-book kernel: [ 3139.802858] ftdi_sio 2-2:1.0: FTDI USB Serial Device converter detected
Jan 18 22:54:30 x3fi-book kernel: [ 3139.802905] usb 2-2: Detected FT232RL
Jan 18 22:54:30 x3fi-book kernel: [ 3139.802908] usb 2-2: Number of endpoints 2
Jan 18 22:54:30 x3fi-book kernel: [ 3139.802912] usb 2-2: Endpoint 1 MaxPacketSize 64
Jan 18 22:54:30 x3fi-book kernel: [ 3139.802915] usb 2-2: Endpoint 2 MaxPacketSize 64
Jan 18 22:54:30 x3fi-book kernel: [ 3139.802918] usb 2-2: Setting MaxPacketSize 64
Jan 18 22:54:30 x3fi-book kernel: [ 3139.804839] usb 2-2: FTDI USB Serial Device converter now attached to ttyUSB0
Jan 18 22:54:30 x3fi-book kernel: [ 3139.804928] usb 2-2: USB disconnect, address 70
Jan 18 22:54:30 x3fi-book kernel: [ 3139.805466] ftdi_sio ttyUSB0: FTDI USB Serial Device converter now disconnected from ttyUSB0
Jan 18 22:54:30 x3fi-book kernel: [ 3139.805516] ftdi_sio 2-2:1.0: device disconnected
Jan 18 22:54:30 x3fi-book kernel: [ 3140.044063] usb 2-2: new full speed USB device using uhci_hcd and address 71
Jan 18 22:54:31 x3fi-book kernel: [ 3141.080076] usb 2-2: new full speed USB device using uhci_hcd and address 72

What could be the cause?
The other arduino works.
Maybe It's something not correctly working? Some parts not well connected?

Are some tests I could try?
Now I'm on a Ubuntu at home. Depends on where I'm (home or work) I can test on a Windows.

Thank's forany help!

Simon