@HAR1122,
A number of things can cause this behavior.
A failing power supply.
Bad component connected to the motherboard or a bad peripheral connected to the PC.
Reseat all PSU connections to the MB and internal components. Try a different, known good, power supply.
Reduce the PC to a minimum configuration. Disconnect unnecessary peripherals, disconnect the HDD and the optical drive, and disconnect front USB ports which includes the USB 2.0 card reader.
Try one stick of RAM in each memory slot. Use all sticks, one at a time, to test all sticks in all memory slots. Try a different , known good, memory module in each memory slot.
No luck could mean the motherboard or the CPU (very unusual) has failed.
Regards