Page cannot be found errors

Using MacBook 3.1 running OS X Snow Leopard, I am getting random "page cannot be found" errors, or the inability to load images, usually on specific sites. Google, Facebook, and Amazon were giving the most trouble yesterday. Now it's happening with more sites. Firefox is my primary browser, but I get same results in Safari and Chrome. Other computers using the same connection just fine (both Mac and PC). My MacBook connects fine when connected to on other wi-fi networks or using my cell phone's 3G.

I have tried clearing all kinds of cache (and in Firefox it's currently overriding to store 0 MB cache) including:

1. User Account/Library/Caches - delete everything inside the caches folder.

2. System/Library/Caches - delete everything inside the caches folder then restart.

according to this discussion: https://discussions.apple.com/thread/2331995?start=30&tstart=0

I've also read through this discussion: http://mac.bigresource.com/OS-X-Leopard-Some-Sites-including-Google-Does-Not-Wor k-w6GTVWfnY.html

Seems everyone else fixes the issue by removing Peer Guardian. I've never heard of that program and don't have it installed (or any other filtering software for that matter).

fwiw, the problem connection is a DSL line provided by Cincinnati-area ISP called zoomtown. (Maybe other Mac users have had similar issues with them?) Obviously I'm going to call the ISP (though I'm not hopeful they'll have a clue) and see if the local Apple Store has heard of this issue with Macs on this ISP. Unfortunately, I probably cannot duplicate the issues at the store.

Appreciate any advice if anyone's had this kind of issue before. Thank you!

Jason