Every day, at a random time, my ancient laptop becomes unusable. Something decides to consume the entire 1GiB of RAM plus several hundred MiBs of swap, and thrash my hard drive. It brings the computer to its knees for 15-30 minutes.

I have set update-manager to never check for updates, I have set update-notifier to not run on startup.

This just happened yet again, and I found update-apt-xapi running, consuming 100% cpu. When it went away, my computer became usable again. Other than that, there's no obvious culprit for this large memory consumption and crippling of my machine.

How can I stop my computer from locking up every day?

This is an ancient laptop with 1GiB RAM, running Xubuntu 14.04