7800GTX power problems

I have recently upgraded from a ATI x300 to a Geforce7800GTX 256 and inturn upgraded from a 350w psu to an ultra connect 500w modular supply.

When I connect both of the 3 pin 12v power connector on my pci express cable to the psu, my pc will not switch on. However when I unplug 1 of them it will switch on and run fine.

I have played Battlefield 2 with all the gfx trimmings with no problems. I am just confused as to why it will not switch on with both cables plugged in.

AND this 1 cable has to be a particular one it will not work with either. In addtion to this after I have tried to switch on my pc with both cables in, I have to remove and refit the fuse from the mains plug to enable the pc to switch back on with the correct cable.

any ideas?

I have a
compaq pressario 1340 AMD64 3200+
4 x 512MB RAM DDR 3200
1 x dvd rewriter
1 x 240GB hdd
onboard sound
ultra xconnect 500w psu
Geforce 7800GTX 256MB

I'm at work at the mo and can't think of any other pc details. let me know if you need something more specific. As I say, it seems to run fine with 1 cable, I am just curious as to why it will not run with 2.

I guess there could be a chance there is too much of a load for your psu and its tripping, therefore not turning on.

i got an Antec 500 True Power 2.0 MODULAR PSU and had to return it because it woudl reset all the time, and was just not very good. Modulars are definetly not as good as non modulars, so i would RMA it and go with another option. I went with the 500W FSP and love it, no problems at all

So either your cable is wired incorrectly or you're putting it in upside down or the psu is wired incorrectly resulting in a direct short circuit blowing the fuse.

Get an OCZ520Powerstream