I've installed a bunch of cat-5 and when I check it for continuity and polarity it seems fine, but when I hook up a pair of laptops with a x-over cord and try to ping between them, they can detect there is something on the other end but I get 100% ping failure (mixed between "hardware failures" and "timeouts"). I punched a few jacks on the end of a length of cable and that works just fine, so I know I can make good connections.
Any idea what might be causing it?
My current best guess is that it has something to do with my the wires being bundled together. How tight can you bundle cat-5 without causing problems?
What it turned out to be:
In short: bad test equipment.
The long version: I borrowed some a better tester and it said that all of the cable was good but I still couldn't get data through it. So I yanked the shortest segment out of the wall (man I love nice spacious attics), re-terminated it and brought it to a guy with a better cable tester and even it said I was doing things right (or at least I was enough of the time that I shouldn't be seeing the failure rate I was). Then I tested the patch cords I was using to do the test... Bingo! They were bad. After getting a known good replacement (and disposing of the old one with extreme prejudice) everything works.
I still don't know why the patch cords consistently worked on a few ports and none of the others but I now have tested everything in the wall and it all works!