Power problem

Hello,
hi when i start up my computer it signal no power on the screen ,you can hear the pc start ,then you hear it died down again,this would go on for about 5-10 min,but onces its up and runing it would not miss a beat it will keep going,also at start up the computer would shut down by its self ,i would push the power button and no power at all so i have to turn it off at the wall then back on and it will fire up again ive change the power leads and still no joy i was told that it could be the power pack and also told it could be the video drivers...............hope this is clear as mud hahaha!