Not sure if this will fix your problem but back in 2001 when I first connected to the ADSL service with BT I had a problem with loosing my connection and having to re boot my computer to get the connection back, so after months of trouble BT came up with a fix which was to open Outlook Express and set to check for email at one minute intervals and leave it minimised at the bottom of the desk top, this keeps the connection active, and this fix has worked for me ever since.
Good luck.