My pc goes to power saving mode after every 20 seconds

hi,
few months back i assembled my pc . But from last 1 month it is giving problem , like before it used to get shut down every 10 min. Automaticly And starts on its own Automatically . Sometime it gets shuts down after 1 hour also there is no fixed period for it . So , finally i uninstalled windows 7 and again i made a fresh new installation of windows 7 . But now after every 5 min. Or maximum half an hour my pc goes to POWER SAVING MODE , the pc will be running but the monitor will show orange light and dark screen. After few seconds the screen will have some options like - 1.start windows normally 2. Start with safe mode . 3. Cmd prompt etc. And before you take an action the pc again turns to power saving mode and this process continues.
I have got the basic configuration i.e -core i5 4670 (using with stock fan ), mobo - gigabyte b85m-d3h , ram 2 gb ,hard drive 320gb. Well next month i was planning to buy a new gpu . Plz help !