Mysterious Boot Problem

[Exaco]

Hello everyone, before i post my question i wanna say that i am somewhat experienced in PC Hardware and know how the system works, so probably i won't have an issue that's simple to fix or diagnose ( but if someone experienced it probably will know what's the deal ).

First time when it occured, around a month ago:
I switched off my PC via the PSU switch, later display didn't showed anything, i've heard the Windows startup sounds, skype etc. So i thought its my HDMI cable or Monitor, but seems like not.
I've opened the case tried to look for loose cables, reinserted the GPU - Same issue, this time no sounds, PC was just rebooting every few secs. So i thought maybe BIOS issues occured, so i just reset it by removing battery - still same, then i've removed the HDD/SSD and the bios screen showed up, i've loaded the optimized defaults, set AHCI mode and saved it, turned off the PC and plugged my SSD/HDD back, after turning on same issue occured, so this time i've removed the HDD only and windows booted, then i turned off and plugged the HDD back - Problem Solved.

Today occured the same issue, but after playing around for like 4 hours i didn't managed to plug the HDD in so it works ( it was plugged in at one moment, but didnt showed up in the My Comp, so i tried to switch the SATA port to another one and the issue persisted so i ended up booting it w/o my HDD after like 30mins of trying.

There was also moments when the PC was turning off in this behavior while i was using it ( rarely ), usually the issue occurs when i restart or especially use the PSU switch.

The issues i am thinking of:
- Electricity Problems in my appartments ( when the pc was turning off in mid of use and after that was barely turning on then electricity disappeared in whole building/neightbourhood and quickly got reset and after that the PC started working like new for weeks. )
- PSU is dying
- Motherboard is dying
- This one will sound funny, but i think its most likely: Cockroaches/Spiders inside my PC damaged Motherboard by causing static charge on some Boot related thing. ( I'm not some dirty guy who does not clean his stuff, this appartment is basically old and has lots of holes in floors etc, so these bastards most likely coming from the basement. )

Issues that's 100% no the problem:
- Monitor/Cable
- RAM/HDD/SSD
- Overheating ( my temps are stable )
- Power issues such as not enough Wattage.

PC Specs:

CPU: Intel® Core™ i5-2320
GPU: ASUS R9 270 Direct CU II
RAM: 16GB Corsair Vengeance® Low Profile
MB: GIGABYTE H61 - GA-P61-S3
SSD: Samsung 840 EVO 120GB
HDD: Western Digital WD Green 2TB
PSU: OCZ 650W

Hopefully it's all clear and i didn't forgot to mention anything also sorry for big ass post and some potential grammar issues

If more details/clues needed let me know.

Any ideas what i should try to do or test ? Without actually replacing parts, this PC is ~4year old btw.