It may sound crazy, but is there more than one firewall active. A friend had a similar Freeserve [Wanadoo] problem, which eventually involved Freeserve and BT who offered many solutions to no avail in the first instance, until finally it was traced to the fact he was running ZoneAlarm and XP, plus one via Freeserve/BT connection. Silly as it may sound,but he was totally unaware of the implications, returning to one firewall solved the problem.
Another fault found was via BT's system outside in-street connection box, and the amount of people that were connected to it. Certain times of the day, the system was in full use, hence poor connection rate.It took three BT engineers visits and checks to find this fault.