But is it assigning a default ip that is totally out of your routers range? ie; instead of 192.168.1.100-110 are you getting a totally different ip per ip segment?
I had this same problem a few weeks ago with both a 2000 and XP system. It was a problem with my Linksys router.
You may be having the smae type issue with your SMC......
I found I could fix it one of two ways.....
Goto add/remove hardware and remove ethernet card and then reinstall it.
or.... do a factory reset of router- turning it on and off/pressing the little hidden button during power on. That seemed to be permanate fix.
Of course- that was my problem on a linksys..... May not even apply here, but your info brought back memories....