It's usually a result of a driver running within the kernel. These threads don't usually show up in
top and even with kernel threads showing, sometimes they don't show. I've been in your situation. There wasn't a way to show the precise cause but upgrading my graphics driver fixed the issue.
I fear the precise problem in your case is probably a regression in the kernel's drivers. My money would be on graphics.
Dislaimer: What I'm about to suggest could break things. It's unlikely but possible. If you're not happy with the command line, or don't know how to get into a LiveCD environment to fix things, I would stop reading here, just in case.
You can fall back to an older kernel (and therefore drivers) by installing the mainline packages. There isn't a simple apt-get for this as older kernel versions aren't built for Maverick. But you can get the mainline .deb files from here:
You'll probably want 18.104.22.168-lucid but you could also try 2.6.36 to see if it has been fixed in later versions.
To test you'll need the packages for your architecture (see
uname -a) and the
-all package. Stick them in a directory with nothing else,
cd in and run
dpkg -i *.deb. You can double-click them to install but you have to do them in the right order or it'll fail.
When installed you'll need to reboot and before Ubuntu starts to load, you'll need to hold the left shift key. This will force the GRUB bootloader to show and you can select which kernel you use.
However if the older (formerly working) version doesn't work, it's a problem elsewhere.