Goes to sleep in 20 minutes. Will not wake up.

Just lost 2.0 TB hard drive of Alienware X51 that had Ubuntu 12.04. Replaced with 4.0 TB hard drive and installed Ubuntu 14.04. Set aside 20 GB of swap space. Left 2.0 TB unused. When keyboard and mouse have been idle for about 20 minutes, computer seems to hibernate. Will sometimes wake up, but it takes at least 10 minutes of periodically moving the mouse or pressing a key to make it wake up. Pressing power button once does not work either. Sometimes it won't wake up at all. Turned off all suspend, hibernate, screen dimming, etc. functions in Ubuntu System Settings. Don't mind the screen dimming, but downloads do not finish unless it's kept awake by using the keyboard or mouse every 5-10 minutes while it downloads.