Is my PSU dead / damaged?

[Sixsixx]

So a couple of months ago I bought an Antec Truepower 750W from a friend who was upgrading, and everything was running fine.

Then today, I tried to add a second graphics card in CrossFire (two 7870's) and the PSU will start for a moment, lights will come on, etc. then shut off immediately after, everything goes out, and it won't boot again.

I'm powering the second GPU from modular cables which the same friend also gave me today.

So, the cables he gave me are:
1x 8-pin to 1x 6-pin
1x 6-pin to 2x 6-pin

After plugging in the 8-pin modular and plugging that into the GPU, it boots fine, but if one end of the 6-pin is plugged in and either one of the plugs at the other end touch the GPU plugs, the system dies immediately, then boots back up a couple of seconds later.

I then get an error from my BIOS saying : Unstable Power Supply detected, Anti-Surge enabled to protect, etc. etc. etc.

My motherboard is the ASUS Sabertooth Z77
CPU: i7-3770K
RAM: 12GB (4x2 + 2x2) 1600MHz
GPU: 2x PowerColor HD 7870

The two things I've narrowed it down to are:

The PSU can't handle the wattage (even though http://www.extreme.outervision.com/PSUEngine says my rig will only require 499W)

The modular cables are incompatible.

Any advice?

 

[JackNaylorPE]

Never heard of a cable that turn a single 6 pin into two 6 pins. That would exceed the PCIE specification.

The Trupower 750 came with the following:
http://www.antec.com/connectors.php?ProdID=23752

1 x 20+4 Pin
EPS 12V
1 x 4 Pin ATX 12V
ATX 12V
1 x 8 Pin EPS 12V
PCI-E 6+2
2 x 6+2 PCI-E
2 x 6 Pin PCI-E
E-SATA
9 x SATA
9 MOLEX
1 x Floppy

It would seem, ya friend shorted you a couple of cables

You should have 4 PCIE cables as indicated in bold above. The 7870 needs "600W power supply (or greater) with four 75W 6-pin PCI Express power connectors recommended for AMD CrossFire™ technology".
http://www.amd.com/us/products/desktop/graphics/7000/7870/Pages/radeon-7870.aspx#2