I am running Ubuntu 14.10 on a desktop computer. Right after re-boot, the computer performs very nicely. However, after a while using it, it starts to slow down because of background activity. Actually it slows down so badly that I can't use the machine. If I wait (sometimes up to 5 minutes) the background activity stops and the computer goes back to normal (very acceptable speed). Any suggestions as of the cause of this and how to fix it?