Good troubleshooting there-most people don't think about the cable possibly being bad. But there are a few more things to consider.
larkost's suggestion is a good one, but if you didn't explicitly configure your DNS servers you wouldn't know if they are now different. To make use of that suggestion, you need to first find out what your ISP's DNS settings should be and then look at what you have. You could also use
Open DNS's servers, which can be helpful if your ISP isn't as on top of things as they could be.
Can I assume you've been using Safari for all your tests? First, go to
this speed test site and find out what sort of results you get. Next, download and install either Firefox of Camino, or both, and try them, both with the speed test and with just any site you're interested in.