Mac suspends and start again

when I stay in my mac pro, closing the lid or even with it open once it is plugged in, it suspends and past according starts and back to stay, but continued to do this. I've done several resets, including alt + command + R + P and failed. Someone help me?

can you boot in Safe Mode (holding Shift key)?

I boot in safe mode and then what do I do?

I boot in safe mode and did the suspension with the screen open and closed.

I boot normal again and the screen is open the same, but with the screen closed light of the apple only gives a small flash that is repeated periodically.

What should I do?

It will be a serious problem? They told me the board power supply and logic board, is it?

Thanks for the help