Greyfeld, do you have a third party firewall installed on your machine? That could be the problem right there. Me and my dad were using a third party a couple years ago with no problems, loved it, and so forth. Then suddenly our internet speeds dropped like a rock. We had broadband (through Comcast), so we were seriously wondering what was going on. We tried everything we could imagine and "determined" that it had to be Comcast screwing us over. Well, it turned out our firewall, on each machine, had changed settings during an update or something, and it was the reason for the slowdown.
If you do have a firewall, I would highly recommend tweaking with it. Firewalls, are apparently, notorious for causing issues. We certainly had our fair share.