I have got the same problem with my NTL connection. I stream internet radio and the connection (upload) stops dead every time at 180kb. I tried changing the ntl proxy to a different server and managed to get 210kb but then exactly the same happened.
Phoned ntl tech support, first time I got India (do not call on a weekend, they are useless and blamed it on my anti-virus). Second time I called during the week and got someone in swansea who went through everything with me (1 hour 30 minutes on the phone), in the end he had no idea and escalated the problem.
Unfortunatley the 2nd level techies got the wrong end of the stick and tried to fix my ntlworld.com ftp account. Third time lucky, I phoned ntl: tech support again (Swansea) who were very helpful, rather than escalating it again I spent 10 minutes on the phone explaning the problem. The techie then found the report from my last call and arranged for an engineer to come and take a look at my ntl: equipment and the ntl: equipment in the street. The amazing thing is, that was yesterday, the techie is coming this evening!
I will let you know what happens as this may help you.
EDIT: First Post!