http://dlsvr01.asus.com/pub/ASUS/mb/...1911_a8n-e.pdf

That is your manual.
CPU Target Temperature which regulates your fan speed is problaby what you are refering to. 54C is where I would want that to kick in at full speed.

Sounds like you are going into hibernation or sleep mode to me. Might want to adjust the time before that takes effect. Yes, you are sitting there reading, but unless to hit a key or move the mouse to reset the timer, yes it will shut down.