I've tried doing an isolation test and it doesn't make any difference. I've tried 3 different modems. Nothing seems to make a difference.
Here are my line stats as reported by my TPLink modem. The error rates are very low so I assume that means I don't have a line problem. Hopefully someone can confirm if they look ok. From my limited knowledge they seem fine. Note that the stat's all need to be divided by 10 (as they are 0.1 dB):
Power State:L0 Downstream UpstreamLine
SNR Margin (0.1 dB):118 120
Attenuation (0.1 dB):270 130
Output Power (0.1 dBm):198 119
Attainable Rate (Kbps):7712 928
HEC Errors:0 54
CD Errors:0 0
LCD Errors:0 0
Any advice on what to try next?