My HD Monitor Says "Frequency Out Of Range"

Okay.. I havent found an answer for this problem..
I have a tower E-Machines computer with a HD Monitor. The monitor is connected perfectly to my tower, which confuses me. When I go fullscreen, on anything.. it does great. Until a couple minutes into what Im watching, and then the screen goes black. And a thing come on the monitor saying "FREQUENCY OUT OF RANGE". So, I am forced to turn off my computer by unplugging it, so that I can restart my computer and watch the same video in a regular size window. It never done this at first, when it was new. Which is what confuses me. Can someone please help me with this problem?
Also, I would like help with another problem dealing with my monitor. After 30 minutes of not touching the mouse, the screen goes black until I move the mouse again. Im assuming it is some kind of energy saver. But I would like to know how to turn off the energy saver. So that I can watch my videos from my bed, without having to get up to move the mouse every 10 minutes. Just to be sure. Can someone help me with this problem as well?

Okay. Thanks.
This really helped. Everything is normal for now. Im just gonna try to keep it that way.