Dell T3500 Auto Turned Off Issue

Hi, My Dell T3500 auto turns off with 5 seconds after pressing the power button. I've tried every possible method shown on YouTube like cleaning motherboard, Ram and checked all the cables....

Sometimes it turns on but while using, it automatically appears a message telling that your system Ran into a problem. 

I've brought this PC to repair person but they couldn't find that issue and advised me to sell it in scrap. It doesn't look that pc has any major fault in it. Mary be short circuit in supply or somewhere. 

Kindly someone help me in this matter.