My desktop is restarting automatically with out the bios

Hi All,
I am using Acer Veriton X4620 Desktop Computer with Windows 7 Professional pre installed. It was working fine until last month.
But, from this month onwards I am facing different issues. After booting it, for some time it works but suddenly it will shutdown and turns back on but the bios screen will not be shown. I thought this was a problem with the mother board (Asrock Killer Z97 OC) and changed it. It worked for a few months and again the same problem. All fans are shut off and then the system also goes down. I have replaced with a new power supply but no use.
What could be the reason? Is there any way to resolve this problem? Any help will be greatly appreciated. Thanks!!
