Mint 13 xfce, AMD 2-core, 16GB RAM, home built pc.
When I have Firefox idleing and I bring up Software Manager,
one of the CPUs goes to 100%, then settles down.
But, if I type a letter in the Software Mgr's Search box,
one of the CPUs will go to 100%,
then dances around 98, 99, and 100%,
and I can't type a 2nd letter
or quit Software Manager normally.
My conky shows the above CPU actions.
Task Manager give no clue
and shows minimal CPU useage.
This has happened other times with other software combos,
but this one I have tested and it happens again and again.
Also this has happened with Zorin OS 6 and Ubuntu 12.04.
So I don't know if it is a basic Ubuntu issue or my hardware.
Wirth's law - Software is getting slower more rapidly than hardware becomes faster.