Assuming you are on a BT Home Hub, Have you tried re-starting it.
Type 192.168.1.254 into your browsers address bar (without http:\\www.)
go to Advanced, system, restart.
If that does not work, try the same, but reset; this will, or at least should, restore to factory settings.
Do you have the same problems wired as well as wireless?
Do you have BT Vision? If so try their helpdesk, I've been very impressed with them in the past.