Third GPU on ASUS M5A99X runs at reduced hash rate

Howdy folks!

Okay, I'm not sure this board is the proper place for a technical question, but since I'm a 'newbie' I'm restricted to here, so I'll just throw it out. Thanks for reading it and sorry it's so wordy.

Here's my problem. I bought an ASUS M5A99X Mobo, it seemed made for mining, with two double GPU slots and one single, I put ATI 7770's in the doubles (Not a great choice, but what I had on hand), and an ATI 6570 (once again, it was just laying around) in the single slot. When I fire up GUIminer, I get around 200-225 Mhash/s each on the 7770's, and 9.5 Mhash/s on the 6570. Disappointing. Once, though, for some reason, I removed a 7770 and the 6570 suddenly ran at around 80 Mhash/s! But when I stuck the 7770 back in, then it (the 7770) ran at 10 or so. Good grief. I ended up removing the 6570, firing up with both 7770's to get my two 200-225 rates back, and then restarted with the 6570, which went back to it's slovenly 9.5. To get at the bottom of this, I've also tried ATI 4890's and another ATI single I can't remember the number of, I've removed things and stuck them back in a dozen times, but always with the same result. Two fasts and one slow.

The dual PCI express slots, when used as duals, are limited to x8/x8 speed, and the single is limited to 4x speed, period. I'm curious if that has anything to do with the problem, but I also believe that all the work stays on the board until the numbers are crunched, so I don't think so. Someone please correct me if I'm wrong.

My question is, to the GUIminer experts especially-- Is this a mobo problem, a GPU problem, or a GUIminer problem? Has anybody run into this or something similar before? A million thanks if you can give me a hint about this. I'm sorry if this is confusing, I know it's a lot of info to pass clearly, and I'm not an english major.

thanks again!
russell