Unreliable POST/BOOT Sequence, Unstable System

[Deil Grist]

Sometimes my system doesn't post, boot, or has an unstable boot (I can tell it's going to be unstable when the Windows login icon stutters), and it's usually resolved by using my reset button to power-cycle the system until I get a working boot. But when it DOES boot stable, it's an amazing machine (parts included in my signature). I selected the parts and built it myself back in March.

Things I've tried: drivers and BIOS up to date, no overclocks, checked and re-seated all power connectors on both ends, tried different GPU power ports on the PSU, and checked PSU voltage through the BIOS and it all checks out. I even took it to Microcenter for them to diagnose and they said they couldn't find the cause nor re-produce the issue (NEVER using that store's service again; they're full of crap). 

Worthy of note: It's perfectly stable and has no issues when a GPU is not installed, but it never boots on the first try with one installed. I've tried a 660, 960, and my current 970; no changes. Temperatures are fine in all scenarios. 

 

I have it pretty much narrowed down to the motherboard or the starting voltage on the PSU (as that wouldn't be detected by the BIOS if it had a stable enough POST to get to the BIOS), but would appreciate any other diagnostics you guys think I should do before giving up and buying a $20 PSU to diagnose with. I would just keep dealing with it, but it's been getting worse lately and I worry about the constant power cycling/improper shut downs damaging my system.

[ManIkWeet]

I even took it to Microcenter for them to diagnose and they said they couldn't find the cause nor re-produce the issue (NEVER using that store's service again; they're full of crap). 

Lol, every store is full of crap...

 

It's a long shot, but maybe one of your USB ports is shorting out, causing some stress on the motherboard that shouldn't exist...