I cursed BTInternet AnyTime for months for its frequent disconnects but decided to check everything at my end before having a go at BT Internet.
This included having my line checked (twice) and buying a new ISDN TA for the Server.
For about three years I had been running the Deerfield/Wingate Internet sharing software to enable multiple access through a Server on my LAN.
Very recently I dismantled the whole Wingagte Server/Client setup and operated two of the PCs independently. Problem solved! And it hasn't returned after re-installing the upgraded version of Wingate.
My point is that if your set-up suddenly changes in characteristics and performance, look to something you may have done recently that may have changed your set-up by over-writing some of the core Microsoft system files.
Try going back to basics for confirmation that your system is still sound. A neat trick I've used in the past is to uninstall a Service Pack and then re-install it. That re-datums your OS.