Sorry to hear about the connection issues you've been having. I've checked your details and can see that you've contacted us since you last posted and that an engineer has been arranged. Please let us know how it goes after the visit so we can help if it doesn't improve.
I thought I'd just follow up on this in case someone else has a similar issue.
The router-to-wall cable has a connection in the middle that's covered with a white plastic tube, so there's no way to tighten the connection if it gets loose without removing the (locked) plastic tube. The engineer discovered that the cable ends inside the tube were barely touching and this was the cause of the problem. He also mentioned that the wrong filter had been installed. All in all it was a poor original Virgin installation. Anyway, after tightening the cables and replacing the filter my connection has been rock solid ever since.