Hmm. Definitely enough power, which may mean that your graphics card is faulty. IIRC, your graphics should still run even without the PCI-E power, but whenever you put load on your graphics card that exceeds the ~75w that the pci-e slot supplies, it will cause a system shutdown.
1 last thing to try though, hook your graphics card up into your computer BUT LEAVE the PCI-E connectors from the power supply OUT. This will mean that if there's any dodgy wiring in the pci-e cables that could be potentially shorting and therefore preventing your computer to boot, you will know about it. If it still refuses to boot, you could try getting a power measurement on the cables. If they are shorting then chances are they've fried your GPU, but best to test the card without the cables in first, and then getting a reading on the cables. If it boots without the cables, replace the PSU. If it doesn't, replace the GPU and test the cables.
(Sorry for the long paragraphs lol)