ye it disconnects and goes to default 800 dpi, sounds like the cable has a short, If you still have warranty send it in if not you might find a cable on ebay or you could ask for one on the logitech forums.
It's most likely the chord (where the cable goes into the mouse seems to get progressively shittier with time). Same thing happened to me until I replaced chord manually with one from an old 310. http://www.crossfire.nu/?x=tutorial&mode=item&id=107
not in mouse
2 what is ur mousepad ?
props to scarzy for this.