Weird Power Glitch

[WindySin]

First, apologies in advance: I know I should search for answers first, but frankly this problem is weird and I wouldn't know where to look.

For ages now, my PC sometimes hangs when it tries to boot. Except it doesn't. The screen never turns on, but it seems to complete it's boot sequence normally otherwise. If I blind enter my password, it seems to log into Windows, and then I can even reboot it from there (I figured this out because my keyboard RGB changes pattern when it logs in). When it does this, the POST lights flash a different sequence (more on that later). The only way I can seem to fix it is to turn it off, unplug it from the wall, wait a while and then turn it all back on again, +/- holding the power button down to hard shut it down first.

It only seems to happen when the computer is left shut down for a while -- longer than about 15 minutes maybe? Almost never happens on a reboot. Never affects the PC in operation. I've resorted to leaving the thing permanently running with the odd reboot just to avoid this. The only error message I've ever seen during the boot sequence is a CPU fan error. When I checked, the CPU fan works fine.

I've got a brickless S4 Mini build, running the following:

HDPlex 400w AC-DC + DC-ATX combo connected to power via a C5 plug
J-Hack M2426 24-pin
Core i7-7700
Asus H270i mobo
- when it doesn't boot properly, it flashes red --> yellow --> white
- when it does, yellow --> green
16GB RAM
RTX 3060Ti

Any ideas? Rough pointers?

 

[confusis]

Interesting - seems to not be CPU, RAM or board related. Unfortunately that leaves GPU - Have you tried alternate outputs or removing GPU and seeing if the CPU graphics recreates the issue?

Red => Yellow => White indicates it's stuck on VGA (GPU)

 

[WindySin]

Interesting - seems to not be CPU, RAM or board related. Unfortunately that leaves GPU - Have you tried alternate outputs or removing GPU and seeing if the CPU graphics recreates the issue?

Red => Yellow => White indicates it's stuck on VGA (GPU)

View attachment 2273

It'd fit the pattern of everything boots but graphics doesn't power on. I'd swear this happened on my old RTX 1060 before I upgraded though, and it never has an issue in operation. I've not tried running it with onboard graphics because unhooking this brickless build is a headache, but I suppose I've no real choice.

What kind of GPU issues might there be? Short of replacing the GPU, I'm not sure how I'd fix this if its a GPU issue. Was hoping there was a config setting somewhere. Also, what do you mean by 'alternate outputs'? As in the HDMI or whatever?