Thought I'd provide an update to this as I've now solved the problem just in case anyone else has had a similar issue. Basically it was the wireless network card that was unhappy and kept freezing the system...I've been through 3 brands (netgear, belkin and edimax) which all failed (edimax was the best but it only could last about 20 seconds).

What I've done instead is to 'trick' the PC by buying a wireless bridge so I can plug the LAN network card into the bridge instead (I'm forced to use wireless for boxing due to having a silly house). The specific bridge I bought was also a print server (which is handy) and is the Netgear WGPS606 Netgear 606 Link.

I can now use my beasty 12gb, quad core, i7 to get down to the serious business of 5 boxing once again...Northrend had better watch out!