Checked the website and your motherboard doesn't have the card to switch around like mine does. I also noticed that your board doesn't have a 4 pin molex on it for extra power...I don't know if this could be the problem just an observation. Do you have any x1-x16 riser cables? If you do you could try the x-1 slot and the main x-16 slot and see if that gives good results.
Thank you for checking into this for me. No, unfortunately I don't have any riser cables (can you recommend a good source? I'd like to invest in a few.) and in terms of power, the connections from the PSU to the cards should suffice. The PSU is equipped with four PCI-E connectors: two 6-pin and two 6+2-pin connectors all together.