Router strange behaviour

[sofi]

When I run Azureus on the router I always have an extra speed of 5-8 MB in the D bar at the bottom and on DU Meter from the download speed it says at the top. I have tested for viruses, worms etc...nothing, when I plug in my old Marconi ADSL modem everything goes back to normal, I am very confused, I have removed the network connection so as to run the router (D-Link 504T) purely as a modem. I have a firewall in place as well as Nat and Firewall enabled on my router. I have restored the factory defaults and reset without any luck. With the cap I am a very worried person as the small file I am downloading could escalate in a lot of megs