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I have Windows XP Home, and recently IE6 has
been giving me a "Page not Found" error after surfing for an estimated
period of 5 minutes. I can shutdown my computer, unplug the broadband modem,
plug the modem back in and boot up my computer, and the browser will work
for another five minutes before giving me a "Page Not Found" error.

This is a repetitive problem......boot...use IE6 for 5 minutes OK..then..page not found then boot again and repeat the process.....over n over nove n over again.

In the meantime OE6 is functioning OK and can even link to IE whilst IE is operative.
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In the Windows add/remove programs I have removed IE.
I have replaced IE with Mozilla Firefox then rebooted.
Opened Firefox and searched the net.
Timed out probably after 7-8minutes
So the problem does not appear to be with IE or Firefox but somewhere else in the systems connection structure, bearing in mind that I do have a constant internet connection in that OE is always functional.
Just thought I add this bit of info to give a clearer picture.

Then, your firefox is OK, maybe some trouble with your ie.