Mystery cabling problem

Ok, here's a good one for the teccy types.

Yesterday I had a call-out to a shop where one of the tills wasn't connecting to the network (XP PC based system). I tested all the cables and found no faults, I was getting a good link to the switch at the till end and yet the till wouldn't connect.

I figured it must be the till, but my tester could see and talk to it's NIC so that looked OK too.

Next I started experimenting with the till next to it, using a 20m patch lead to see what happened when I plugged till 1 into till 2's data point etc. I eventually concluded that it must be the patch lead that was causing the problem, even though it tested OK, so to prove it I connected the problem till to it's own data point with the 20m patch lead and it worked fine.

Concluding that I had resolved the problem I went and got a brand new patch lead out of the van and.... nothing, no connection. I thought maybe I had a dodgy lead so I took the one off the neighbouring till (a 'known good working' lead) and still nothing. So I plugged the 20m lead back in and it connected immediately.

So the question is this, why would a 20m lead work when a 2m one won't? The only potential clue I have is that some of the wiring for the security tagging system runs up the same pole into the ceiling as the cable for the data point for this till, and every time I plugged/unplugged a patch lead it set the tagging alarm off (which is odd, I assume there is also a problem with the tagging system, but I don't know anything about it so I didn't start messing with it).

Stupid question, but were you connecting the patch lead to the same data point (assuming a wall patch point), or directly to the hub?

Also, are there mains supplies going up the same pole? If so then it might be picking up some interference (assuming you are using cat5e patch leads, these can be susceptible to interference from mains, especially on poorly shielded stuff).

And finally ... as has happened before ... on the 20m lead, did it auto-negotiate the speed down to 10Mb/s rather than 100Mb/s?

tagging cable should be ok, if the controller for de-tagger is remote then only data runs down the cable. the de-tagger on the till will emit interference, down aswell as up so make sure nothing too close.
what alarm went off?
there's no connection from the detectors on the door to the till.

It's quite possible ... and the fluke might show 100mb, but when into the hub it might drop it down due to the extra distance. I have had similar before where it would drop to 10mb and work fine, but when running at 100mb there was data, but it was that corrupted that it wouldn't work

Its not a crossover cable is it? Not all network adapters auto detect crossover cables.