Can't connect to cable modem (Ubuntu 10.04 LTS)

Hi. I've just installed Ubuntu 10.04 LTS on a new computer and can't get a working connection to my cable modem. The connection is with an ethernet cable from the motherboard (Gigabyte GA-MA74GMT-S2) directly to the cable modem (Motorola SB5100) supplied by my provider, Telstra. I've tried another ethernet cable with no change. My other computer, a dual-boot system with Windows and Hardy Heron, has no connectivity problems. I've copied some settings from the latter to the new computer without success - see below.

Telstra requires a static IP. The settings they supplied me with include an address, a mask, a gateway and a DNS. These I entered as described at https://help.ubuntu.com/10.04/internet/C/connecting-wired.html and with that done, Ubuntu recognised a connection, showing at the network manager icon in the top right. This icon has since disappeared. (Why?)

I can ping the address, but not the gateway, nor the cable modem at 192.168.100.1. Both the latter give Destination Host Unreachable.

This new computer replaced an older, which had a working Dapper Drake installation and internet connection. The connection was routinely swapped from one computer to the other by swapping cables and resetting the cable modem. The cable modem's web interface shows that it supports two different MAC addresses, and it has learnt the address of the new computer.

Here is some more information about my setup:

/etc/network/interfaces:

auto lo
iface lo inet loopback

auto eth0
iface eth0 inet static
address 203.97.116.36
netmask 255.255.255.0
gateway 203.97.116.1

(This initially contained only the two loopback lines, and wasn't modified by the network manager gui mentioned above. I copied the eth0 section from my working Hardy Heron installation.)

route -n:

destination gateway genmask flags metric ref use iface
203.97.116.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
169.254.0.0 0.0.0.0 255.255.0.0 U 1000 0 0 eth0
0.0.0.0 203.97.116.1 0.0.0.0 UG 100 0 0 eth0

ifconfig eth0:

eth0 Link encap:Ethernet HWaddr 1c:6f:65:50:1e:ff
           inet addr:203.97.116.36 Bcast:203.97.116.255 Mask:255.255.255.0
           UP BROADCAST MULTICAST MTU:1500 Metric:1
           ...

Thanks to anyone who can suggest a way forward.