PPPoE drops

[blimpyboy]

pfSense: 1.2.3-RC1 embedded
h/w: Alix
ADSL Modem: Vigor 100
WAN type: PPPoE (dialondemand/idletimeout/periodicreset all unticked/blank)

• All working fine for months, but for past 3 wks loses link roughly every 2-3 days.
• By 'losing link' what I mean is no Internet connectivity for LAN clients (eg can't ping 4.2.2.2)
• Once it's in this state, WAN (vr2) shows in GUI as:

Ststus: up
    PPPoE: up [disconnect]  <- pfSense thinks link is still up!?

Hitting 'Disconnect' then 'Connect' brings everything back to normal… for 2-3 days then back in loop ;-(

I should point out that my customers ISP (BT in UK) informed them that 'they would be doing some line work, but it won't affect you...' just prior to this all starting.  Very suspicious, but I just KNOW the onus is now on me (from my customer and their ISP) to prove it's not my pfSense/Vigor100 hardware.

Before I start doing things like replacing Alix/Vigor with fresh units, might there be something in a log somewhere that might shed some light.  I've looked in /var/log/system.log and it shows something similar to this:

Point A (eg 9am Sun)
  ... reams of normal stuff...
Point B (eg 10pm Mon) - details of say a PPTP VPN connect from me
  ... nothing, at least that appears relevant to this issue ...
Point C (eg 9am Tue) - details of my vr2 disconnect/connect from cust room when they called me!
  ... goto A

In other words there appears to be nothing much in the log between everything being fine and me having to do a WAN disconnect/reconnect.  It's as if pfSense is completely unaware that there is (or has been) a break in the link.

Due to the 2-3 lag time between events, this looks like it might be very time consuming to resolve.  Thanks in advance for any ideas that might lead to a speedy resolution.