I have a NOWS SBE 2.5 Server (I know its no longer supported... update due in August)

Installed 3 years ago and generally just works.

2 weeks ago started getting a problem where the Internet would stop working on the network
Checked all PC's on the network with Virus checker, Replaced Router, Switch Cables, got the ISP to check ADSL line etc...

On friday discovered the problem is caused by the NOWS SBE Server - disconnect and everything works fine
Plug the NOWS back in and within minutes internet dies

Replaced Network card on the server but still the issue.
even pinging the Router from results in massive ping failures

Network tools shows 14Gb+ transmitted on ETH0
Router data flow monitor shows 2000-3000 sessions coming from the server
The Router Draytek 2960 has Danial of Service option which is on (tried it with off too)

Others have suggested a Virus might be responsible - possible ??

How does one find out whats causing this ?