Ubuntu 10.04 LTS does not assign an IPv4 address to eth0

This is fresh install of Ubuntu 10.04 LTS on an Intel ICH10 machine (quadcore), a Fujitsu Esprimo P5731. It uses the Intel 1000e onboard network adapter. Internet connection is established through a DSL router/modem that does NAT, so for all local PCs internet is just traffice that goes out over the standard gateway, no PPP is involved.

Between 1 out of 3 and 1 out of 2 times after bootup, the network indicator says that network is up, but it isn't. Clicking on it reveals that the IP address is 0.0.0.0 which is, of course wrong. Initially I set the system to use DHCP, which worked fine during installation and a couple of days after that.

Then came the first occurence of the problem: IP was 0.0.0.0, according to the GUI, but subnet mask, routing and DNS were set _correctly_. Going onto the shell and inspecting that with ifconfig eth0 revealed that the system had an IPv6 address assigned (where from? The router doesn't issue IPv6 address leases, and Ubuntu is, by default, set to ignore IPv6), but _no IPv4 address_ whatsoever!

I tried to configure the setup manually with a fixed IP over the GUI - to no avail! No matter what IP I configure on the GUI, ifconfig consistently says that no IPv4 address at all has been set...

But I found two different ways to circumvent this problem:
a) Manually configuration using ifconfig
b) Disconnecting the PC from the network and reconnecting.

So Ubuntu's eth0 autoconfiguration does work - but not during boot. And the manual configuration on the GUI does not work at all.

There are no problems with the router or the router's DHCP server. Two other PCs with Ubuntu 8, an OS/2 and all sorts of Windows systems run flawlessly. Also, the problem does not occur on every boot, but sometimes 1 out of 3, sometimes 1 out of 2 times. Really annoying...