Wierd Connectivity Problem

I am a SBC Yahoo! DSL subscriber. I have the 2Wire router with 1 Ethernet, 1 USB and WiFi.

My home network consists of an older Win98SE machine, connected via USB, and two Win XP laptops, connected with Ethernet and/or WiFi.

Sometime back, the Win98SE machine stopped being able to browse (IE and Firefox) consistently with DSL (one or two pages OK, but then "Page Not Found"), but dial-up worked fine. I tried everything and ended up reformatting and re-installing. It was all good for a few months, but now is doing it again!

(Originally, I also had a Windows 95 machine, connected via Ethernet that suddenly exhibited the same symptoms at the same time as the Windows 98 machine. I never got around to reformatting and re-installing that one, but it did work for a while with Ethernet).

I have tried:
Norton 2004
Spybot
Thorough Scandisk
Remove and Rebuild Communications
Remove and Rebuild Winsock and Winsock2

I dread having to reformat and reinstall again, particularly if I need to do it over and over again.

Any ideas?

Thanks!