I don't think its your PSU. If you want to eliminate your PSU as the culprit just run the miner with reduced intensity, basically run it at 50% and see if it runs then. It will use less power and you can see if its your issue or not.
Try what I suggested and get your motherboard manual and find out about the available lanes on your PCIe slots.
The problem was solved when I downvolted from 1150mV to 950mV, now my rig is perfectly capable of mine with 6 gpu.
Now I am just reducing voltage to reduce costs, so I downvolted to 875 which is the value I have found on some settings that other users recommended.