[Jimmy-J]

I can think of a couple of things it could be...

Firstly is everything seated properly in the motherboard? GFX card/RAM/sound/Network card/etc. and are all cables connected firmly?.

I'd unplug all cables and peripherals that aren't needed when booting the PC.

One of the most common issues is heat, Overheating, Is the fan on the CPU heatsink working as it should be? It might be going around, but is it working at its optimum RPM? It may need a clean, or a fresh drop of thermal paste on the CPU.

Try connecting the monitor through another port, VGA/DVI/HDMI or DP

Also are the fans on the GPU doing their job?

Could be a failing CMOS battery? Is the date and time correct when you enter the BIOS?

Could be a faulty stick of RAM or motherboard/GPU