intermitent uploading

uno uploads routinely and then without any good reason fails to upload, instead, produces the following messages.

using debian squeeze 64 bit on oracle vm virtualbox uno /dev/ttyACM0

avrdude: error reading signature data for part "ATMEGA328P" , rc=-1 avrdude: error reading signature data, rc=-1 avrdude: stk500_rec(): programmer is not reponding

dmesg produces:

Product: Arduino Uno [ 1378.622136] usb 2-2: Manufacturer: Arduino (www.arduino.cc) [ 1378.622137] usb 2-2: SerialNumber: 649353431333512181A2 [ 1378.622214] usb 2-2: configuration #1 chosen from 1 choice [ 1434.465614] usb 2-2: USB disconnect, address 6 [ 1450.308116] usb 2-2: new full speed USB device using ohci_hcd and address 7 [ 1450.774444] usb 2-2: New USB device found, idVendor=2341, idProduct=0001 [ 1450.774447] usb 2-2: New USB device strings: Mfr=1, Product=2, SerialNumber=220 [ 1450.774449] usb 2-2: Product: Arduino Uno [ 1450.774450] usb 2-2: Manufacturer: Arduino (www.arduino.cc) [ 1450.774451] usb 2-2: SerialNumber: 649353431333512181A2 [ 1450.774935] usb 2-2: configuration #1 chosen from 1 choice douglas@debian:~$

Thank you