Vista shutdown problem

Ive been having a very strange issue thats been happening more frequently.

my windows will randomly shut down, not like a "poof computer is off" shutdown..but just all of a sudden I will see "windows is shutting down" for a few seconds, and computer is off.

On top of that, it doesnt finish the windows shutdown procedure, as if I was doing a shutdown manually, it takes much longer. this is more like you clicked shutdown, then just held the powerbutton while it was trying to shut down.

Im assuming its not a hardware/PSU issue, because why would vista actually try to shutdown windows if it was a hardware problem or PSU dying. Seems Id just get a normal sudden power down, or lock up if that was the case.

Im out of ideas what could cause this, anyone have any clue?

if in happens when you PC idle check your power saving options