aDSL performance problem

OK, here's a weird one, which I cannot understand.

We used to have a relatively slow (64K baud) leased line. We now have a
much, much faster always on aDSL connection (512K baud outbound, I think
2Mb inbound but frankly the inbound doesn't matter for this).

We have two sets of users in different remote offices. One set connects
to the internet via a shared modem, the other via a shared aDSL
connection.

Since we changed to aDSL at the server end, both groups have
intermittently reported that just one user in their office (not always
the same one) is getting abysmally poor performance from the POP server
(a few tens of bytes per minute). Each time this happens, the user can
reboot their PC and it still won't help - they continue to get abysmally
bad performance, while another user at the next desk using the same
shared dialup connection gets good performance.

Also since we switched to aDSL, we've had independent reports from
different users of Web applications which have been performing well,
suddenly, in the middle of a session, starting to perform diabolically
badly, and that this can only be fixed by rebooting the /client/.

The mail server is running Debian 3.1, and is regularly updated. The POP
daemon is qpopper 4.0.5-4sarge1. The primary Web application server is
also Debian 3.1, but we have today had a report of exactly the same
problem with a legacy server which is still running Red Hat 5.1, because
the Web application in question is written in PHP2 and there isn't the
budget to port it to anything more modern.

None of these servers is heavily loaded; they have plenty of RAM and
aren't swapping; they have plenty of disk. I've think I've eliminated
anything in /server/ performance as being the cause.

So far in all the cases this has been reported the client has been some
version of Microsoft Windows. So it /could/ be a Windows networking
problem; but I'm more inclined to suspect a network, firewall or aDSL
problem, as we had no reports of this sort before we switched from the
leased line. The firewall is a 2wire 1800HG.

--
(E-Mail Removed) (Simon Brooke) http://www.jasmine.org.uk/~simon/
;; Our modern industrial economy takes a mountain covered with trees,
;; lakes, running streams and transforms it into a mountain of junk,
;; garbage, slime pits, and debris. -- Edward Abbey