Do you have anti-virus installed which includes a link scanner or similar? Try disabling it!

I've had some major problems, similar to what you describe - everything connected to the internet bar my browsers (IE, Firefox, Safari and Chrome). I scoured the internet for solutions and tried just about everything, including a clean re-install. I even switched from AVG to Avast following the re-install but still replicated the problem. Another feature of the issue was that I couldn't even kill the browser process from the Task Manager: the only effective cure being a reboot.

Today I nearly pulled my remaining hair out as none of my browsers would access my own website - particularly helpful as it's just gone live. Even rebooting didn't work - whichever browser I used simply told me it could not connect to the site - I knew this was unlikely as everything on the server was intact. I tried to access the site from another machine (runnning XP) and, hey-presto, it worked with no problem at all.

I tried resetting winsock etc, uninstalled Vista service pack 1 and ZoneAlarm but still couldn't connect to the site from my Vista machine. By this time I knew the site was definitely still online because I received an email from someone via the website.

I then attempted to connect from a third (also XP) machine on my network, this time, I was also unable to connect. I checked all my network and firewall settings - they were fine.

Finally, after more searching of the internet, I found a forum where the concensus appeared to be that the problem was probably related to security software, AVG was mentioned. This reminded me that the main difference between the machine which would display the site and the two which wouldn't was the link scanner in AVG and the Avast Web Shield were active on the PCs which wouldn't show the site and disabled on the one that would.

I deactivated the web components in question and connected to my site straight away.

I don't know if this will resolve the intermittent browser disconnect, but I'll try and keep you posted.

Hope this helps (even if it meant putting up with reading how miserable I've been the whole day!).