No device found on COM9

Hardware Arduino Due
1

On a new Due, Windows 7, Sketch is Example Blink, Arduino 1.5.2.

I have installed the serial driver and the Device Manager shows it as 'Arduino Due Programming Port (COM9)
It does work once in a while but then it reverts to 'No device found...' when I add a Serial.begin(115200) in the setup().

What is the magic?

2

I have got the same problem and I also don't know why.