iMac Mid-2011 sometimes boots into login screen instead - and after sleep Pages/Numbers sometimes displays weird characters

My folks have an iMac mid-2011 that they picked up from a relative. I do not have their computer with me, but here are the overall specs:

iMac mid-2011
iMac12,1
Mac OS X 10.7.5
2.5 GHz
4 core

[1] Sometimes, the iMac will boot into the login screen, while other times, it does not. The device is set to log in automatically. The iMac is shut down as normal, e.g., Shut Down in the Apple menu, so it shouldn't ever be showing a login screen.

[2] Sometimes, after the iMac wakes up from sleep, documents already opened in Pages/Numbers can display weird characters all over, and the only way to fix this is to restart Pages/Numbers. Other apps such as Safari and Finder display as normal after a wake up from sleep

What could be causing these problems?

[reader50]

Are they backed up?

I'd look at the SMART values for their hard drive. The boot-to-login sounds like a boot file isn't consistently reading - you might check the boot sequence in Console for errors after one of these incidents.

Also boot into diagnostics and do a system check, especially of the RAM. If HD and diagnostics look clean, I'd reinstall the OS.