network becomes unreachable

Hi there,

I have a Fedora core 6 on my new pc. I don't know if it matters, but I
have to set the MAC address of my ethernet card since my ISP sticks to
my old computer's one. The problem is the following: an hour after
system startup, network becomes unreachable. Even

# ifconfig eth0 down
# ifconfig eth0 upû
# dhclient

does not help. What helps is

# /etc/init.d/network restart

Do you have any idea what could have gone wrong?

- Gergely