I had a similar problem that I managed to trace down to a bad hub.  If you can
borrow a hub from someone else, see if that does the trick; alternatively, if
you have enough spare parts, you might make a jack-to-jack crossover cable so
you can temporarily replace the hub between a pair of systems -- if they can
then talk, it's the hub that's at fault (be really careful constructing the
crossover cable so you won't feel entitled to blame it ;-).

An alternative debugging strategy, if you can't borrow a hub, is to
temporarily snake a new cable between your failing system and the hub,
assuming that you can do this easily (i.e. no drilling) without exceeding
the length restrictions.  Otherwise, just replace the original cable and
see if that helps.

(In my case, I only had trouble sending data from one computer (an iMac)
which incidentally was the furthest away from the hub, so of course, I started
out by replacing the cable to the hub a couple of times, until I got suspicious
and moved the iMac to a different room to try a different jack...)