@coinits You should try to hook up a differnt psu. Simple thing to do and you don't need a big one, just enough to run 2 cards to verify if its the psu or not. 450-500w should do it.
What kind of risers are you using?
I'll try the PSU. I have a Corsair 750 that works fine. There are no risers. I have a GA-X79-UD3 from Gigabyte which has 4 slots.
Are your cards ones that are mb power only?
If so that might be the issue. Too much power on the mb