Infuriating web problem - caching?

Hi all. I'm the reluctant 'computer guy' at our small firm and I am currently creating and maintaining some basic web pages for our site. Most are simple html pages created or edited in dreamweaver but with .asp extensions as the original site has most of its pages protected by a login and this is the way the original webmaster built it.

I've noticed a really odd recurring problem lately. Sometimes I get an old version (like, a week old) of our pages, and any newly uploaded pages return a 404.

The ftp side of things has been tested and is functioning correctly. I can access the site just fine from other machines on the LAN.

Something appears to be caching our site within my PC, and is delivering me an old version of the site (like, a week or more old) whenever it feels that way inclined. When the problem happens I'll see the old version of edited pages, be unable to load any of my 'new' pages, and the problem will continue for maybe several minutes to an hour or so. A forced refresh makes no difference, nor does using Firefox or IE. Then it will go away, I'll be able to load the full site and see the modified pages, no problems. Until next time.

I've cleared 'recent history' and 'offline storage' in Firefox, 'browsing history' in IE8 and the 'Offline Web Pages' and 'Temp' folders in C:\WINDOWS on my machine and it doesn't change anything. Also, it doesn't explain why I'll happily see the new version for most of the day then suddenly be 'locked out' of new pages and see only old ones.

Forced refreshes will see the images rebuild, but the text of the page remains the old version and new pages refuse point-blank to load.

I'm wondering where the old data could be hiding, what is causing it to be displayed (our internet connection is generally pretty stable with no dropouts that I've detected) or whether this is some kind of misconfigured .asp 'dynamic' content screwup on my part that is refusing to update a page on my machine.

It is really bizarre to suddenly be shot back in time to a version of the website that is weeks old and I can't figure out how it's happening.

Any ideas?!