every so often I'll leave my computer on for several hours and come back to it and notice that the screen has turned off but it hasnt gone into sleep mode. this doesnt always happen there are time when my computer will sleep when its suppose to but I just cant figure out why it doesnt always do it. I have my screen saver to come on after 10 minutes the screen to shut off after 30 and its set to go into sleep mode in an hour but like last night when I went to sleep with my computer on and woke up hours later only the screen was off. does anyone know the cause of this?