Or DSL,
With a router?
ISP's often reassign dynamic IP address once a day around midnight to 1am. Some routers that have backlevel firmware versions do not handle this too well. Their network keeps dropping your confused router. You have to power it off long enough for it and the ISP's network to forget your MAC connection information. Usually at least 15 minutes. Updating the router firmware usually helps solve this problem. As does getting the ISP to "update" the local node controller that you are connected to, with your correct hardware configuration and that you have XXXX router.
Without a router?
I don't know about XP, but there are a lot of Microsoft updates for networking that could be effecting your systems ability to adjust to a dynamic IP address change. Do you have all of the Windows Updates? Have you tried running WINIPCFG and doing a Release All and Renew All command.
With Dail-up?
I have heard of problems in some locations with Earthlink dial-up service, where if you stay connected for too long at one time, their busy network will drop your connections priority and not let you back in until it resets, if they have too many users connected. I ran into the problem when their local server became over loaded when they picked up a failed local ISP's customers. It forced me to a Cable IPS. Now they have additional lines in the area and it is no longer a problem, when I am force to use it.
In addition to phone line problems, I have seen home security alarms, caller ID boxes, fax machines and answering machines cause problems on the phone line with dial-up connections. Sometimes they can be time of day dependent as to when they are putting noise on the line. Problems often start after an electrical storm in the hot summer months.
