With such intermittent problems, I would want to do some hardware tests to make sure the hardware platform is stable, before playing with software settings.
Ideas:
- run dmesg and look for suspicious messages during boot
- run memtest overnight
- run fsck on the hard drive or SSD
- boot a different OS (or siduction) from USB stick or CD ROM -- play with the user login and root login
- use smartmontools to test the hard drive, if there is one