Line drops & Upstream reduced

Having endless problems, compounded by a dreadful level of service from the ISP. Line is about 3km from exchange & runs the business adsl max - set up 2 + years ago, generally runs at 4+mb d/s and 832 u/s. For past month or so have been getting frequent (10+ per day) line drops & upstream that is often at 352! Have switched filters, tried 3 different routers (diff chipsets), changed drop cable, used the test socket. Even removed the phone & left that line on a full divert with BT.
BT keep claiming it must be end user equipment & ISP does nothing.
Log from the current router (Speedtouch585) is below - I've removed IP addresses but it shows the speed/margins etc at boot up & then within a couple of minutes typical drop out & relink at slower u/s speed. Happens even in middle of night when office unoccupied & no computer equipment on at all!
Anyone any ideas of (a) cause, (b) how to interp the log esp what are the 3 different CHAP challenges about? and (c) how to resolve it?
Thanks

00:04:35 (since last boot) xDSL linestate up (downstream: 4160 kbit/s, upstream: 576 kbit/s; output Power Down: 19.5 dBm, Up: 12.5 dBm; line Attenuation Down: 49.0 dB, Up: 27.5 dB; snr Margin Down: 15.5 dB, Up: 6.0 dB)
00:04:22 (since last boot) FIREWALL event (1 of 11): enabled rules
00:04:22 (since last boot) xDSL linestate down
00:01:14 (since last boot) FIREWALL event (1 of 1): disabled rules
00:01:14 (since last boot) PPP link up (Internet) []
00:01:14 (since last boot) PPP CHAP Chap receive success : authentication ok
00:01:14 (since last boot) PPP CHAP Receive challenge (rhost = lns15.the.dsl)
00:01:14 (since last boot) PPP CHAP Receive challenge (rhost = edin2.lts)
00:01:13 (since last boot) PPP CHAP Receive challenge (rhost = ERX2.Edinburgh2)
00:01:10 (since last boot) xDSL linestate up (downstream: 4160 kbit/s, upstream: 768 kbit/s; output Power Down: 19.5 dBm, Up: 12.0 dBm; line Attenuation Down: 49.0 dB, Up: 27.5 dB; snr Margin Down: 15.5 dB, Up: 6.0 dB)
00:00:58 (since last boot) xDSL linestate down
00:00:40 (since last boot) LOGIN User logged in on [HTTP] (from )
00:00:37 (since last boot) xDSL linestate up (downstream: 4192 kbit/s, upstream: 768 kbit/s; output Power Down: 19.5 dBm, Up: 12.0 dBm; line Attenuation Down: 49.0 dB, Up: 27.5 dB; snr Margin Down: 15.5 dB, Up: 6.0 dB)

In some ways hope it got worst. I ended up with my upload dropping from 900+ to 500 then 200 then 100 then finally 56 -- intermittently.

When I checked there was very audible noise on the line. The good news is I was able to report this as a BT voice fault, and after 2 visits and some cable swaps it appears now fixed, and my broadband is back to normal.