PC sometimes doesn't boot

[ventrue]

Problem 1:

When I turn on or reboot my PC, only fans and drives spin up. Nothing else happens. Booting within 2 or 3 hours after shutdown seems to be significantly less likely to work. Once it's running, no further problems occur, as long as I don't reboot.

This has gotten much worse over the past two weeks, but has been occuring from time to time for years (!).

Resetting or turning off and on until it works is my current workround. Might work the first time or take 50 tries. Turning off power completely or simply waiting might have an effect.

 

Problem 2 (Just for completeness's sake, not the focus of this thread unless related!):

Two or three times over the past weeks, I've had a display flicker, show artifacts and ultimately turn off.

Over the past few years, I also had problems with display recognition after booting, with one display usually being recognised only after turning it off and on again and usually at most two displays working simultaneously. Persisted after GPU change, but seems to work fine now with the newest GPU.
 

Already tried, with no effect:

Using only one of the two RAM kits (both work)

Using single RAM sticks (all are working)

Resetting BIOS (Problem occurs even with the very moderate default settings)

Parallel GPU and CPU stress tests (no crashes, no artifacts, temperatues stay within limits, CPU_max=63°C, GPU_max=76°C)

Several hardware changes over the past few years that would, in combination, theoretically rule out just about everything. Which doesn't really help.

New GPU and reintroduced sound card in the past month, so at least that shouldn't be the cause

I'm ruling out anything driver or Windows related, because this is happening pre-boot.

 

Specs:

Corsair RM850
--Asus Sabertooth 990FX R2.0 (with most current BIOS)
----AMD FX8350
------Noctua NH-D15
----16GB DDR3 RAM, consisting of:
------2x 4GB Corsair Vengeance, 1333Mhz/1866Mhz, 9-9-9-24/9-10-9-27, CMZ8GX3M2A1866C9
------2x 4GB Kingston HyperX Fury 1866 Mhz, probably 10-11-10-29, labeled as HX318C10FBK2/8 but identifies as KHX1866C10D3/4G
----Sapphire Radeon RX580 Pulse
------3x HP w2207h, 1680x1050 at 60 Hz
----SoundBlaster X-Fi XTreme Gamer
----3x HDD
----1x SSD
----Corsair K95 RGB, single USB plug
----Logitech G5
----TrackIR
----HP C4180, not powered
----USB-Hub
------USB-Stick
------Saitek Warthog HOTAS, equals two power hungry joysticks electronically, currently both powered off at the USB hub
----4x case fan, 1x CPU fan

 

Changes to specs in the past:

Put sound card back in (new drivers, yay!)

Different GPU

(anything below happened months or years ago)

New SSD (old one broken and replaced)

New PSU

Less RAM (The Kingston kit is the new one)

Another different GPU

New SSD (old HDD remains)

Removed sound card (no drivers for Windows 10)

New PSU (Corsair VS 550 or something like that)

New CPU (Phenom II X4 810)

New Mainboard and RAM (Don't remember, was the Phenom's original AM2 board)

(I believe that the problem existed even back then, though very rarely)
 

Some sensor data:

Electrical, idle, mainboard:
+12V - 12,004V
+5V - 4,996V
+3.3V - 3,288V
Vdda - 0,52 -
Vcore - 0,96...1,245VV
VIN4 - 0,036V
VIN6 - 0,792V
3Vsb - 3,288V
VBat - 3,336V

Electrical, idle, GPU:
GPU Voltage/VDDC - 0,744...0,75V
GPU VRAM Voltage in (+12V) - 11,516...11,521V
GPU VID - 0,75V
GPU Current - 1,367...1,635A
Core Current - 1,5...6A

Temperature, idle, mainboard:
CPU - 44°C
Motherboard - 36°C
VCore1 - 49°C
VCore2 - 46°C
NB HT - 45°C
DRAM - 37°C
USB31 - 44°C
USB32 - 40°C
PCIE1 - 44°C
PCIE2 - 39°C

Temperature, idle, GPU - 39°C

Temperatures HDDs - 41°C, 30°C, 32°C
Temperature SSD - 39°C

 

All SMART data is ok, HDTune results are attached.