I don't know where to start with my problem. It happens intermittently and is annoying.

The problem: Gmail won't load or download attachments.

Here are the computers I've tested the problem with:

iMac 10.6.7 fully updated with Safari 5.0.5

MacBook 10.6.7 fully updated with Safari 5.0.5

Win 7 notebook with Firefox 4 latest version

iPhone 4 

I have reset/restarted all my software/browser cache/settings/router and still the following problems occur. I do not have manual DNS set up on ANY of my computers as far as I know - they all report DNS 10.0.1.1 which is the address of my router, and the router does not have a manual DNS setting.

The problem is that Safari loads Gmail but the activity meter continues to "spin" while it is loading the x out of x item in the status bar. It eventually gives up.

When I open a message and try to download the PDF attachment, nothing happens. It never makes it to my download window.

When I click "View", it tries to redirect me to "https://docs.google.com/viewer?........" but it stalls while it tried to load the PDF/viewer.

This happens on BOTH the iMac and the MacBook.

On the iPhone 4 I can use the Mail application and open the document to view without issues.

On the Win 7/Firefox setup I can DOWNLOAD the attachment, but if I click VIEW (ie. Google viewer) it stalls trying to load "https://docs.google.com/viewer?......."    

My next step was to tether my computers to my iPhone 4 cellular data provided by Vodafone 3G.

This solved ALL the problems, and each of the computers was able to do as it should in downloading or viewing the PDF. I noted also that Safari managed to load the entire Gmail page without getting stuck on "loading x of x items".

What's going on here? This problem seems to plague me every few months or so and resolves on its own.

Any ideas? I'm frustrated.

Edit: For the sake of clarity I'm not using any adblockers. On my Macs I have no antivirus software installed. OS X Firewall is running I think, and the router is Airport Extreme with default security options, nothing fancy. The Win 7 notebook has MS Security Essentials running but nothing else.