See
http://forum.kitz.co.uk/index.php?topic=15087.msg281445#msg281445This is not just a Fibre or even a 21CN issue, I'm seeing classic congestion symptoms (single stream must slower than x6) on 20CN Max Premium.
There's a significant fact here: Max Premium has a higher priority on the BT network. So that leaves one conclusion - it's Plusnet.
Oh, if anyone suggests it's a line fault causing a high error rate, my current stats:
Uptime: 5 days, 13:10:12
DSL Type: ITU-T G.992.1
Bandwidth (Up/Down) [kbps/kbps]: 832 / 8,128
Data Transferred (Sent/Received) [MB/GB]: 752.71 / 4.34
Output Power (Up/Down) [dBm]: 12.3 / 19.9
Line Attenuation (Up/Down) [dB]: 11.0 / 20.0
SN Margin (Up/Down) [dB]: 15.0 / 6.5
System Vendor ID (Local/Remote): TMMB / ----
Chipset Vendor ID (Local/Remote): BDCM / TSTC
Loss of Framing (Local/Remote): 0 / -
Loss of Signal (Local/Remote): 0 / -
Loss of Power (Local/Remote): 0 / -
Loss of Link (Remote): -
Error Seconds (Local/Remote): 26 / -
FEC Errors (Up/Down): 0 / 0
CRC Errors (Up/Down): 16 / 27
HEC Errors (Up/Down): - / 24