New Linksys WAG200G + download problems ?

I've finally got an ADSL modem/router and have done away with my USB ADSL modem. I went for the Linksys WAG200G and have connected it up in wired mode with wireless disabled. Installation went smoothly and now have both my PC's connected to it, and both have Internet access.

The problem is that the download speed seems to fluctuate quite a bit (ie. fast one moment and then slower the next - up and down), and when downloading multiple files at the same time using a download manager one of the files seems to get the bulk of the bandwidth and downloads at high speed, while the other files download at a much slower speed.

It came with firmware 1.01.01 but I have upgraded it to the latest 1.01.05a fw. The only settings I have changed from default are my time-zone, Keep-Alive, the router password, ISP username + pass, Wireless disabled and the VPI/VCI.

What could be causing this fluctuation in download speed and favouring of a single download ?