Line noise could be one potential problem, but if you were working OK just a few days ago, then I doubt that could be it.
In certain cases, the bandwidth on your ISPs end is "fractioned" off depending on how many users are connected. Thus, the more users the less available bandwidth pool you will have access to. In addition, if the modem on their end is running at a certain speed, you will not be able to exceed it, and yes I've seen ISPs run with 14.4 on the receiving end.
All in all, some possible things to try if you are able:
Try the computer in a different phone line
Obtain driver upgrade for your modem
Upgrade to broadband
Chances are, if you no configuration changes were made on yoru computer since this started occuring, then it is most likely not on your end. You could also try calling the ISP, letthing them know what you have done so far, and if possible, monitor you as you connect.