Question Pc problem!

[LtTuran]

I have a system;
MB:F1A55 r2.0
Cpu : amd athlon II x4 631
power suply:500w
8 gb kingmax
1 ssd 120gb and 1 hdd 180 gb
video msi gtx 760.
Problems:
After i start the system after 3 secunde shut down , i press again after 3 second same think.I tried everyrhing ..... but i tried one more think i remove the 4 pin power(12v) from MB near cpu and then turned on after 3 second i put back the 4pin and the system is working like normal.
What is the problem.?

 

[rgd1101]

make and model of the psu?
so the system working now?

 

[LtTuran]

make and model of the psu?
so the system working now?

i have a spacer psu 500w ,even with better psu is doing the same thing.After i shut down the pc wont start only if remove the 4pin w8 for 4 second and plug in and the pc workin well