Gradually increasing packet loss

I've also posted this to the adslguide forums if anyone wants to check
for replies to that first: http://preview.tinyurl.com/3369fz

My sister recently started having ADSL troubles after having an ADSL
connection for a few years. She used to have a Zyxel 650H-E1, which
stopped working for some reason. She changed to a Speedtouch 585v6, it
initially wouldn't connect. Logging in with bt_test@startup_domain made
the Speedtouch work again (for a while - see below). Possibly related?

Line: Pipex Solo 250
Router: Speedtouch 585v6, firmware 6.1.4.3.0
ADSLNation XF-1e filter, 0.5m ADSLNation rj11-rj11 pro+ cable. Also
tried a different cable & filter.

Here's a recent DMT screenshot from when the connection is working fine:
http://www.k.eclipse.co.uk/dmt.png

Here is the graph which l8nc produces after the router has been on after
a while: http://www.k.eclipse.co.uk/graph.png
(RIP l8nc)

Here is a DMT screenshot from ~ 2pm today - the router has been turned
on for 1 day 9 hours+: http://www.k.eclipse.co.uk/dmt2.png

My sister says that when her connection gets slow, it can be brought
back to life by going into the router and using the "check connectvity"
option, which makes the router work okay until 30 minutes or so later
when it happens again. It is not a router overheat issue - I have put
heatsinks onto the hot chips, and drilled a grid of vent holes in the
top of the case.

I will be trying a Draytek 2800VG later (when Draytek sends me a
replacement PSU for it).

What could be behind such a graph? Any ideas?