I'm trying my hand at building a Litecoin mining rig with three graphic cards connected to the PCI-E slots using raiser cables. Unfortunately I have some problems with getting the motherboard to detect all the graphic cards. And it is not even consistent as it sometimes detects all three GPU's but mostly only find two of them. It seems to be the one connected to PCIE5 (the slot farthest from the CPU) that isn't detected.

I know this is the kind of problem that is really hard to pin down so I don't expect any quick solution, but any pointers to what might be failing here would be very appreciated.

I'm using the following HW:

Asrock 970 Extreme4 motherboard
AMD Athlon II X2 250 CPU
XFX ProSeries Black Edition 1250W PSU
3 x Sapphire Radeon HD 7970 3GB GDDR5 graphic cards (no SLI/Xfire used)
Kingston DDR3 HyperX blu 1333MHz 8GB RAM
Kingston 60GB SSDNow V300 SATA3
Microsoft Windows 7 Home Premium EN
all in an open plastic box together with a bunch of fans and an ASUS USB-N53 wifi adapter.

I have switched around the GPU's and raiser cables and found no fault so I think they're all working. And the GPU temperature stay around 70-75 degrees Celsius when running so heat shouldn't be an (immediate) problem either.