Silly question, as you're obviously an experienced user, but have you performed a "hard reset" of the router back to factory default settings?
I have the same router but it wasn't supplied by Virgin, I bought it myself from Amazon and it's actually running old firmware - v2.25 from May 2008. On the basis that "if it ain't broke, don't fix it", I haven't updated it.
I have 2 desktops (Win XP/Vista & Win 7), Laptop (Win XP), Netbook (Win 7) and Mac OSX all connected to it, so far very reliably. The 802.11n WiFi adapter bundled with it is installed in the Win 7 desktop and connects consistently at 300Mbps.
It may be that, if yours was "customised" by Virgin, there was more than just their firmware installed and changing the firmware hasn't restored it to a conventional retail version.
I have a "Virgin" Netgear WGR614 (freebie from Telewest) and it isn't possible to install updated firmware from Netgear's site in that.
I know this isn't much help but I do like D-Link routers and prior to getting the DIR-615 (for its 802.11n support), I was using their DI-524 which was totally reliable.