1st Dec 10 - 6:47
You may well find the box that your cable goes into is oversubscribed, we were told should be 200 connections in each bit, we were 247!!! all to do with when it was NTL, had exactly the same trouble, it only happens when everyone gets in from work and starts using internet, only happens in evenings.
Request an engineer then ask him to move your slot in the box in the street, worked for me, i'm now at the top and its all fine.
hope this helps a little