Your figures seem to be well within the limit. Although you are showing some errors and these would increase if the cap is lifted, you should be able to get a better speed than you are. Also, your errors seem to be coming in spikes, ie., 39 in the last 6 mins out of a total of 227. This suggests to me that something in the environment is causing the errors, rather than just a bad line. This may be something in your house switching on or something outside like street lights for instance.

I can only suggest that you get on to CS and try to get put through to Tier 3, who are the only ones who can lift your cap. Before calling plug into the Test socket, as they will almost certainly ask you to do so. Remember the Test socket is behind the faceplate of the Master socket.