Ok, here's my story
2500K Sandy bridge @ stock/4.3ghz
Club3D 7870XT, overclocked
8 GB Kingston XPS 1600
Asus Xonar XT
Samsung 120g SSD
WD 3TB HD
Corsair 600CX & Tagan 480
The problem is the following:
1. When I start my PC after having left it off for about an hour and it POSTS, it will hang when booting windows, no BSOD, USB devices lose power.
2. After resetting, it usually gets into a boot loop. Starting, no post, restarting, no post etc.
The problem can be completely and 100% solved (meaning NEVER hang in windows and ALWAYS post) by:
1. In problem 1's case, I go into the BIOS, wait a minute, don't save and exit. This makes it boot just fine
2. in problem 2's case, I unplug the power cord for about 10 seconds, plug it back in, and it will boot just fine. Without unplugging, it will stay in boot loop mode.
3. The problem can also be solved by loading optimized defaults, and not overclocking. Not AT ALL. So heightening the multiplier by 1 (from 3.3 to 3.4 ghz) already causes this issue.
Once the PC is running, there are no issues whatsoever. It has never crashed, it runs superPi at 4.3ghz without any issues, can play games on it all night long. When it boots into windows and doesnt hang right after login, it will stay rock solid.
What I have done to try to fix the problem:
1. New PSU. Obviously the first thing I tried. Didnt help.
2. New Motherboard. Got the same motherboard so I didnt have to reinstall windows. Didnt help, exactly the same issues occur.
3. Booting without audio and video card, nope
4. Bios: tried F10 (no video from video card), F16 and F18i, all the same behaviour.
5. Loading optimized defaults: works fine until I touch the multiplier, then problems start again.
The reason I dont believe there is anything wrong with the system like Bad memory, unstable overclock etc. is because it is rock solid when it boots. If you have any suggestions at all, I'd love to hear it!
(one more note: I have a fan controller with a little powered screen. On every unstable boot (meaning one that doesn't POST or one that is gonna crash windows) I always see it blink several times during the startup procedure. Not blinking is pretty much an indicator of a good boot. This made me suspect the power supply, but after replacing that the issue didnt go away. It's still weird though...)