Hi, sorry you had so much up problems with the update
Check this article I made (or see my signature) for info about how a Firefox 3.0 bug with Xorg causes a lot of CPU usage to occur.
1.) Did you install fresh or upgrade?
2.) What wireless driver are you using? If you need to know you can right click the wireless icon in the notification area (top-right by default) and select "Connection Information". It's listed as Driver.
3.) Are you playing music and using PulseAudio? If you want to try disabling PulseAudio, open Synaptic Package Manager (System->Admin.) and remove the pulseaudio package. Then press Alt+F2 and execute gstreamer-properties (remember you can press tab in the dialog to auto-complete text). Select a different Plugin, such as ALSA, for the Default Output. Restart any applications using sound, such as Rhythmbox.
4.) If you're using an Intel Graphics card (such as most notebooks that don't have an Ati/Radeon or nVidia/Geforce logo somewhere), using desktop effects and using 24-bit color depth. Attach your /etc/X11/xorg.conf configuration.
I think we're going to need a lot more info to figure out where the CPU usage is coming from.
When this happens, or in a session after it has been happening, open a terminal (Applications->Accessories->Terminal) and execute the following command and then attach process.times to this thread.
ps -e > ~/Desktop/process.times