When I worked for ISP, one of our steps seemed incredibly silly but none-the-less it sometimes seemed to fix problems like these. Presuming it is possible (I don't know your network topography) you can try it... short of trying another cable. Basically the step was to swap the ends. For arguments sake, I'll pretend your cable goes from PC to router. Take the end that is in the PC, put it in the router. Take the end that was in the router, put it in the PC. Might sound strange but I have seen it work, short of replacing the cable entirely.