A couple things.
Under the settings tab there should be an option for " enhanced USB networking". try to enable/ disable this.
Also, it is possible your USB port is no longer functioning properly. A search for USB Problems yields MANY results, some of the fixed include simple things such as reflashing a new rom/radio , to the extremes of having the USB port replaced on the mainboard.
Also, have you tried to sync it to more than just 1 computer? Try on a couple. Make sure your activesync isnt the cause.