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.
So what's the culprit here? Looks like you have limited power supply, if reducing the voltage works then it's the power, I'm glad you are able to figure things out, can you share the exact watt that your GPUs are pulling? Thanks