What PSU and what CPU are you using? I understand you believe power is not a concern but if your rails on not properly balanced you could be overloading one of them. PSU & CPU model?
Of course it would be ideal if you could slot that fourth card on board w/o an extender to test.
ST1500 PSU
So I had to get creative since the last slot is not that well suited for a double-wide graphics card...
AN installed card not only covers the on mobo power/reset/OC buttons but also interferes with the case wiring...
Anyway, after shortening the pins on mobo and cutting the connectors in half, I got it in there... it a tight fit.
Everything works fine...
Conclusion:
1 - recieved a bad extender cable...
2 - the molex splitter does not draw the same power as the card would from the mobo. and this probably causes issues I've observed... I may be 100% wrong on this one, it's just a hunch... Someone else may be more knowledgeable to explain.
Thanks!