HomePlugs & xbox random mac hardware address & IP problem

HI I have an old xBox set up as a media centre on my home network
using ethernet homeplugs (like these:
http://cgi.ebay.com/ws/eBayISAPI.dll...m=110362435788)

Everything works well but my Dlink (DIR-655) router registers the xbox
as a different MAC address every time it is turned on. This makes it
annoying to FTP into the Xbox because the local IP given by the router
is constantly changing and I cannot reserve it because the MAC address
is random each time the xbox loads.

Anyone got a solution for this or have a similar problem with
HomePlugs?

Thanks a lot,
Ciaran