Hehe, me few weeks ago, same situation …
After checking everything, only reason left - motherboard broken.
Asrock 970 Ex 4 ;)
Got a new one in exchange and voila it works.
By the way, what CPU are you using ?
I got an AMD Sempron 145 and sometimes got problems with the PCIe “Lanes” while running 5 cards on one board …
So check different PCIe Slots and use Linux, Windows did not work for me …
[url=http://www.cryptobadger.com/2013/04/build-a-litecoin-mining-rig-linux/]http://www.cryptobadger.com/2013/04/build-a-litecoin-mining-rig-linux/[/url]
Cheers.