Frustrating internet problem

[GiganticDays]

Setup:-
Windows 7 Ultimate (Phenom II X4 9650)
Gigabyte GA-MA770-DS3 V2.0 with integrated Gigabit LAN
Belkin F7D1401 ADSL2+ Modem/Router
Netgear ProSafe GS108 Switch
ISP : TalkTalk Business (up to 24 Mb down)

I'm having intermittent web browsing issues with the above setup.

Sometimes I can't load web pages. I though it was a connection thing but if, while the page is trying to load, I ping a web address I get the normal responses.
I though it might be a DNS thing but I'm not pinging IPs I'm pinging URLs and they work.

I've tested the CAT5 cables connecting the router and switch and they're OK.
I've swapped out the router for the one supplied by my ISP - no joy.
I've even tried using alternate DNS servers and nothing changed.

Any ideas?

[GiganticDays]

Might have a clue here.

I have a number of virtual machines set up using Oracle VirtualBox.
Some of them use the host NIC in bridged mode.

If I disable the VirtualBox Bridged Network Driver then the problem seems to go away.

Edit: Looks like that was a red herring. Problem remains. Thinking of a new ISP