Video Card Compatibility

I have a Precision T7400  64bit running Windows 10 pro, 32 gb of RAM with dual E5440 Quad core zeons and a Quadro FX 3700 video card. The BIOS is updated to A11.   I tried replacing the card with an EVGA GTX 580.  I tried it in both of the PCIe 2.0 x 16 slots.  It requires both a six pin and eight pin power connection.  The T7400 has a 1000w psu. I used power plugs P5 (six pin) and P19 (eight pin).  I also tried the P18 with the P19.  No matter what I do, when I try to turn on the computer, the power button flashes amber indicating a power supply or power cable problem.  I checked the voltages on the eight pin connector and they were as designed (12v on pins 2 & 3 and 3.3v on pins 4 & 8 with the remainder being COM).  I also checked the P5. I am sure it is ok because I use it with the Quadro.  I don't have any other plugs in the machine that provide 3.3V.  What are my options, or am I missing something?