Have you looked at the 'processes' tab to see what process is actually taking the time on the fully loaded core?
You might be better off using htop in a console window as it offers a better instantaneous representation of CPU and memory states.
From the console as root or sudoer:
Code:
apt-get install htop
Or you can use Synaptic and search for 'htop' and install it that way.
Once installed open a terminal [Applications>Accessories>Terminal] and issue:
The default sort order in htop should show you the highest consuming process at the top of the list. It will also show you the load-average figures which are arguably a better indicator of the CPU state.
/EDIT Does anyone else know how much bandwidth the Ubuntu client viewer uses? If it's sending full pages at a high rate, it could consume a lot of bandwidth when in use.
A trick for derchris to try would be to log in via SSH only without the graphical client and run bmon (probably need to install bmon first).