Vista - Intermittent & bizarre loss of web site access

Setup.

Cable Modem

Hardwired PC downstairs, direct into router
Two PCs upstairs, into switch thence to router
One PC upstairs, wireless
One networked printer, also into router.

The "Two Pcs" are my Vista Ultimate x64, and a testbed running W7 RC1; these have a DVI switch to share the monitor

Every now and again I am unable to even ping sites such as news. bbc.co.uk, and many others.

Yet, if I switch to the PC downstairs, or even the other one by my side, these sites can be pinged and opened in a browser.

Sometimes this will go on all day, and then seem to resolve itself. Other times, it disappears and I am fine for another couple of days.

Nothing seems to point to any pattern of activity that might be causing this.

Any ideas? It's beginning to piss me off...

By the way, this is ALWAYS the error (Firefox text, same error in IE & Chrome)

Connection Interrupted
The document contains no data.
The network link was interrupted while negotiating a connection. Please try again.

Hi Jeremy, I'd be interested in your solution as I'm having a similar issue, although mines on a laptop with wireless connection to my ADSL router. Another laptop (wireless) and a PC are also connected to the router, but they don't have any problems. I too have tested different browsers.

Hope someone is able to help us out

Yes - I found that an IP filter file I had downloaded for P2P work was being picked up by the traffic shaping software I also use. Removed it and all well.