Blank 16U2 programming

Forum,

Need some advice. I have two boards that are clones of the UNO.

On one I can recognize the 16U2 as an UNO comm device. This board however occasionally comes up in windows vista device manager as unknown.

On the second it programs the 16U2 but it never recognizes this, in windows vista device manager, as an UNO comm device and lists it as device unknown.

I have checked the usb connections and using rich gammons software I can read and write the parts all day long through the ICP port.

If I didn't know better I might guess their is a power up issue.

Any help or guidance would be appreciated.

Thank you

Mike

Sorry Nick Gammon