Try this: clear the CMOS by shorting the pins on the MOBO while turned off.  Then use one single GPU to test every slot, then connect one by one progressively, with restarts in between.

That is how I have got that mobo to work with 6, but I have never got it to work with 7 gpu, maybe because I am using windows 8.1

I would really love if someone could help me with another issue you might encounter in the future.

For some reason, after a couple of hours mining perfectly, one gpu drops its mem clock from whatever stock setting it is to 300, which causes that gpu to mine a 1/5 % of the speed, usually around 47 sols/s (amd 470 4 gb modded). so I am constantly having to reinstall drivers, patch, etc, to get it work normally again, since simple restarts do not solve the problem.