I'll throw some things out that may or may not be food for thought. I'm not up on these things though.
Graphics cards are notorious pigs on the 12v rail.
I know there is some practical limit to how much amperage to draw from a single power supply lead (including any splits). I Googled it just now and for a 4pin molex I see 5amps on each the +5v and the 12v rails, 25watts on the +5v and 60watts on the 12v. This is what I stumbled across in case you want to ponder it:
Can't guarantee how accurate that is but it's a place to start. Seems like splitting a single lead and plugging both into the graphics card could easily be inadequate. If you have an adapter you could use with a different supply lead, and plug your 4pin in directly, that might be worth a shot.
Another thing is, a power supply with lots of wattage can still be too weak on the 12v rail for a given card. The sticker on the power supply should tell you how many amps/watts it has available on each voltage.
Other thing is, some power supplies often used to have the 12v rail split to accommodate the higher draws ("Dual Rails"). I don't know if those are still around. I avoided them like the plague, I don't know how a person knows which leads are on which rails. If you've got one of those though you would want to use a power supply lead from each of the 2 rails.