hi, i am experiencing the same thing. i would jiggle the cable, and i could tell from my PC that it was disconnecting / connecting.
i initially thought it was the USB port on my NC. it turns out, it actually IS the NC USB cable itself. the simplest way i can describe it is that the inside of the plug on the cable is very "hollow", so it doesn't bind well against the black plastic part inside the port on the NC.
the USB cable for my cell phone has a very similar form factor, yet the inside of the plug on the cable is "narrower", and so has a better bond when plugged into the NC. i can jiggle my cell phone cable all i want, and it stays connected no matter what.
i've taken a picture to show the difference: http://www.flickr.com/photos/wuziq/6696342611/
hope this helps some people resolve the mystery.