Ubuntu – PC freezes after long period of inactivity (~> 30 min)


My Ubuntu system is configured to turn off the screen and lock itself after 10 minutes of inactivity. (System settings —> Brightness and screen lock).
If I leave the PC for about 10 mins or 15 mins, I just move the mouse and set the password, and I'm good to go. But if I leave my PC for let's say 30 mins or longer, then when I try to wake up the system by moving the mouse, everything freezes and the only option is to reset.

My system is Ubuntu 12.04 LTS
and my pc is a Acer aspire 5750G

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