Maybe your housegrid, the electricity in your house is not okay.
Maybe there is some sort of "problem" in the current.
To much fluctuations in your house current.
to much or to little voltage or ampere, or its not always 50 Hz.
Dont take the electricity in your for granted!
Try removing the CPU from the socket with power removed from the board. Then clear CMOS (clear RTC) and place a single memory module in the slot furthest from the CPU. Install the CPU and mount the cooler and then see if the board will POST.