Firstly this is not complaint or support question - just noting the experience I had when one of the memory modules went bad. This might help others when they start experienceing unexpected behavior. Normally memory is not the first thing you think of going bad when your computer does not behave as expected. Especially in my case, it was just 3 months old memory modules upgraded on 2 years old laptop.
My laptop running ubuntu 14.04 started acting up sometime during last month. Coincidently this started happning after reboot after last update. The update also had new version of firefox for me to blame. I could login, run shells fine, but firefox kept crashing mysteriously. When I tried to update system again, I got a red scary circle in the indicator panels saying updates failed. I even had syntax error package list computer maintains. I tried cursing new updates and fixed syntax error manually with no luck. Things slowly worsen over the period of time. Now laptop started rebooting on its own with no apperant reason. Sometimes when rebooting, I was getting kernel panic.
I decided to reinstall OS with the assumption that somehow, my OS is completely corrupted. Checked the health of hard disk and everything looked ok. During installation I kept getting errors about source not matching. Checked the checksum of downloaded ISO and they matched.
After multiple retries, system installed, but did not work as expected. Tried again with new imaged burned on new memory card. Same problem again. Decided that my memory card had reader or writer gone bad and use a dvd. Burned a dvd with same problems.
At this moment after 5 reinstalls of 14.04, I decided that chances of all my input devices going bad are very low. So problem must reside somewhere else. Memory test from the same iso reported the problems in first. Searched for the old memory modules that I had saved after upgrade. Installed them in the laptop again.
Now I am back in business with everything running very smoothly.