Some NTL staff might say this is due to faulty equipment, i suggest its excessive traffic on the equipment and you should talk to support, and ask for line 2 support, and then book an engineer....
Get them out to look at your perfectly fine cabling and - sooner or later if enough people do this they will have to take note, they will be forced to accept that their network isnt coping and maybe fix the proxy/cache servers.
just my 2p mate
