Computer randomly turns on and off

Hello, I recently built a computer using these components:

Intel Core i5-3570K Ivy Bridge 3.4GHz
Antec BP550 Plus 550W Continuous Power ATX12V V2.2 80 PLUS Certified Modular Active PFC Power Supply
G.SKILL Ripjaws X Series 8GB (2 x 4GB) 240-Pin DDR3 SDRAM DDR3 1600 (PC3 12800) Desktop Memory Model F3-12800CL9D-8GBXL
ASRock H77M LGA 1155 Intel H77 HDMI SATA 6Gb/s USB 3.0 Micro ATX Intel Motherboard
Western Digital Caviar Black 1TB SATA3 6GB/S 7200RPM 64MB
Gigabyte GeForce GTX 550 Ti Windforce 970MHZ 1GB 4.2GHZ GDDR5 DVI HDMI VGA PCI-E Video Card
NZXT Source 210 -Inches ELITE-Inches Midtower Case with 3.0 USB - White

I assembled all the pieces and firmly connected all the wires, and had this little problem. At first, the computer turned on and read the mobos BIOS just fine. Then the next day, when I turned the computer on, it worked and turned off like a second or less later. The whole thing turns on and then off, even the PSU and the fans. After the PSUs internal fan stops, they start working again, and so the process is repeated. I was very worried about it. After switching the PSU off, it did this for another cycle or two, so the capacitors must be working. Once I turned the computer on again after another try, it worked just fine and read the hard drive by saying "no bootable device". After this I gained hopes once again... But then the cycling thing happened again. Has anyone else encountered this problem? Its pretty frustrating. I have a hunch its a problem with either the PSU or the mobo. A friend of mine told me to test the RAM, but it clearly wasn't the problem. Some help would be appreciated, thanks!
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More about computer randomly turns
  1. I would remount the motherboard, but first, pull off all the power supply leads. You may have a short in the board, or something underneath. You may also have a metal piece from the backplate touching one of the ports. Check the backplate and bend or break off any leftover metal.
  2. you used standoffs between the mobo and the case right?
  3. This is my first build, and I didn't get any plastic washers or anything so I had assumed I did everything right. After reading your responses, I put two layers of tape over the holes in the case for the mobo and poked them with a screwdriver. After putting everything back together, its back to working order. Thanks guys, you're life savers.
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