Browser keeps forgetting how to get to websites...

I have a strange networking problem - my browsers seem to keep forgetting where certain websites are, but just for a couple of minutes at a time.

So I can be happily browsing a website, then suddenly it will just start timing out and I won't be able to access at all for 2-3 minutes, then all of a sudden it's back again like there was never a problem.

It's not a network outage, as I can successfully access a bunch of other websites/email/chat during the website outage. Nor is it simply a server issue, as it can "lose" several websites at the same time - and most of the one I have problems with are the ones I use most often (including google), and most of these are my own websites which are hosted at different companies. Also during these outages the email to the affected servers often craps out at the same time (and the mail server is a physically different machine to the webserver). Also, most of these websites are mine and have live tracking, so while I'm unable to browse the website I can see that other people are browsing the website successfully.

Any ideas as to what might cause this?

Also, while the browser is timing out on the website I can successfully ping the domain using the domain name - confirming that a) the server is up, b) I have a network connection to it, and c) that the DNS is correctly resolving the domain name to the IP address...

Also, the various websites disappear at the same time, and reappear at the same time.

So google and one of my sites were just inaccessible, but both came back simultaniously.

Also, although it tends to be the websites I visit most (or maybe I just notice it because it's the ones I visit most), it's not the same ones every time. One time it might be 1 site and another site but everything else is fine, and others it will be maybe 3 different sites, but aain all the others are fine (even those that failed last time)

It's just like it's randomly forgetting how to find websites for 2-3 minute intervals regularly throughout the day.

And it's not browser specific.

Can no-one really not even suggest a place to start? Some diagnostic steps to at least round down the problem area?