attenuation?
3.5 km line, 44 to 45 dB attenuation noise from -2 (night) to 6 (day) sync at 6400 ATM. Until about 12 hours ago it was 6688 but it's got cold, wet and windy in the last couple of days...It does seem odd that 47 dB attenuation is quite so poor, I'd expect a lower speed of 2 Mbps or above and upper something under 4 Mbps.
I think Huge needs to dig about and see if he can get the current information out of the modem and look at the telephone wiring/filters etc if he hasn't already optimised that.