Sounds like you've pinced the cable. Could be o/c or two wires s/c. s/c is easier to test (if you have a multimeter). Just disconnect both ends and then at one end try various permutations to see if any pins are shorted. Checking for o/c is harder. If you have a spare socket, you could wire it to wrap back the connections, allowing you to test for o/c from just one end.
Alternatively try taking the upstairs comp downstairs and plugging it directly into the router, just to prove that it is the outside cable thats duff. If so, then replace the cable without worrying about testing it.
