I've got talktalk (wired) and normally it's very good but sometimes it goes iffy. It'll keep dropping the connection and then connecting again, and keep repeating every minute so you can't really have a decent surfing session or download anything. It doesn't do it that often, but whenever I ring up to complain I get some numpty in mumbai blaming my setup or my modem and keeps telling me to reboot the modem or delete cookies (like that's going to help) - I tell them it's a problem with the line but there's nothing they can do as they're just reading from a script and that's as much as they can do. After I get to the end of their script I get a flag set on my phone number that allows me to ring second line support which is apparently in the uk, without going through to mumbai, but I've given up waiting in the queue after about half an hour. I suppose that's why it's cheap... only £21 a month compared to BT's best part of £40./.
But the first time it happened I was using my BT modem and I had to tell talktalk to send me one of their modems and that worked. Then last time it happened again on my talktalk modem, so I plugged my BT modem in again and that worked... what changes I don't know.