Need help diagnosing boot problems...

How it started
My system has been running pretty much fine for the past three months (since I built it). About a week ago, my computer was showing the blue screen when I woke up in the morning, so I turned it off and back on by holding the power button. When I turned it on, I heard a spark sound and smelled burning rubber. The fans started for ~1/10 of a second and shut off. The computer then refused to start. After testing the PSU and concluding it was dead, I sent it back to EVGA and received a replacement yesterday. Unfortunately, I'm still having issues.

The problem
When I try to turn on my system, the fans start momentarily (~1/10s), then everything shuts back off. Presumably the system is asking the PSU for too much power. I then have to switch the PSU off and on to see this effect again - I guess it goes into some kind of lockdown mode.

When I remove the video card and plug the HDMI cable into the main I/O shield, the system starts up but gives no video output from the onboard graphics. I have no idea whether or not it actually boots, but the system does not shut down after pressing the power button again - I have to hold it, suggesting Windows hasn't actually loaded.

The green light on the motherboard is on whenever the PSU is on.

Things I've tried
-Removed video card (see above)
-Followed this thread to the best of my ability.
-Unplugged everything from the power supply except CPU and motherboard cables
-Unplugged all USB devices (they don't seem to get power anyway)
-Tried using DVI cable instead of HDMI for monitor (no effect)
-Unplugged all fans

Things I haven't tried
-Replace PSU cables (the PSU is modular, and I'm still using the original, pre-RMA cables)
-Replace motherboard or CPU (not convinced either one is dead)
-Unplug CPU liquid cooler
-Take system apart
-Unplug any memory (pretty convinced this is not the problem)

My ideas

Here's where I need help, badly - I'm pretty much at a loss as to what's causing the issue.

When the PSU died, it's conceivable that multiple parts downstream were damaged, but it's hard for me to diagnose what's broken. The cables may very well be the problem - maybe there's a short circuit in, for instance, the PSU-mobo cable that causes the system to use almost 750 watts, so that the video card pushes it over the limit. The cables don't seem to get really hot though.

Or maybe there's a short circuit somewhere on the motherboard, causing similar problems. Again, I looked for hot, smelly, or scorched-looking areas and found nothing.

The CPU could also have a short somewhere - this would explain the iGPU problem, the failure to boot, and possibly the (apparent) overload when I plug in the GPU.

Lastly, multiple parts could be broken. The instant shutdown with the GPU plugged in seems to suggest card issues, but then again the system doesn't work without it either.

None of these explanations seem satisfactory to me, and I don't know what to try next. Should I get new PSU cables? RMA the motherboard? Buy a new CPU? I'm really at a loss.

Any help would be greatly appreciated!

Part List
CPU: Intel Core i7-4770k (delidded - no warranty coverage)
Cooler: Corsair H100i
Motherboard: ASUS Z87-A
Video Card: Galaxy Geforce GTX780 HOF
Power Supply: EVGA NEX750G (750W, more than enough juice - system should be <600W)
Memory: Corsair Vengeance 3x8GB
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  1. Looking at your components i think you're overclocking? Did you do a clear cmos too?
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