Hmm, I think you might have more complex problems that I suspect, but have you tried the following as root in a terminal:
/sbin/ifconfig eth0 down
/sbin/ifconfig eth0 10.0.0.8 netmask 255.255.255.0 broadcast 10.254.255.255 up
/sbin/route del default
/sbin/route add default gw 10.0.0.2 eth0
10.0.0.8 - the IP you want to assign to your machine
10.0.0.2 - the IP of the gateway / router that gets you to your ISP to have internet access
This is, to completely take eth0 down, and then get it back up on a fixed IP address...?