Powered Riser: Rig shuts off w/ molex connected and works w/o. Why?

[sleepless]

Hello,

I have a really strange problem which I'm trying to solve since I have risers.

I have 4x R9 280X and an Enermax Platinax 1500W! So the power shouldn't be a problem. My mainboard is an Gigabyte GA-990fxa-ud3.
I am using exactly this kind of risers: http://coincable.com/wp-content/uploads/2013/10/IMG_32322.jpg

Well, the problem is that whenever I use all four GPUs without molex everything works great. The Platimax has two kind of connectors. The first one is 12v6 and the second one is 12v4. When I connect the molex to 12v6 it shuts down after only 2 minutes of mining. If I use the 12v4 it lasts at least 10 minutes, sometimes even two hours. In both cases (12v6 or 12v4) the whole system immediately shuts down without bluescreen or whatever and I have to turn the I/O on the PSU to O and than to I again to start it.

And: I tried to connect every riser to an extra slot at the PSU because I thought it might be too much power for one single slot. But: Same issue.

I really don't get it. I've lost so many hours trying out and haven't found any solution or at least a logical reason why it would work unpowered rather than powered.

So I'm asking you: What could it be? Please help me!!!

Thank you

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[Duane Vick]

If you have to reset the power supply, I'm guessing that there is a short or low-resistance path being created by connection of the molex cabling. Possibly bad PSU.

[richmke]

Your psu has 6 rails. That means you have to balance the power usage over those 6 rails. Using the molex may be overloading that rail.