Sure your molex adapter isnt faulty. If the molexs work themselves, stands to reason, they should run through an adapter. Try diff molexs?
In anycase, realize the GT 280 calls for minimum 550W, personally I'd want straight draw, and would get one with a 8-pin, but thats me.
Penewah!
Minimum of a 550 Watt power supply.
(Minimum recommended power supply with +12 Volt current rating of 40 Amp Amps.)
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