This is caused because apt's cache is too small to handle all of the packages that are included with stable, testing, and unstable. This is also very easy to fix. Add the following line to /etc/apt/apt.conf
APT::Cache-Limit "8388608";
Several months ago I created an article with 5 APT (Advanced Packaging Tool) tips for both Debian and Ubuntu available here. APT is the package manager in Debian and Debian-based distributions, like Ubuntu. Here's part two of that article, with 5 more tips and tricks for APT.