Different Performance with identical equip/software but different motherboards..

[Scared]

Can anyone shed light onto why I get 38 mhash more per 7970 running on a Gigabyte MoBo (GA-990FXA-UD7) than I do on a MSI 890FXA-GD70.

Here's the Hardware -
  4x Saphire 7970 - Mem 685 - Core 1200
  AMD Sempron 145
  2GB 240-Pin DDR3 SDRAM DDR3 1066
  Barracuda SATA 60 Gig Drive
and the Software (Not that this should have an impact)
  Windows 7
  Diablo Miner
  MSI Afterburner
 
I get 708 mhash per 7970 after 24 hours on the GA-990FXA-UD7
I get 670 mhash per 7970 after 24 hours on the MSI 890FXA-GD70

This accounts 154 mhash loss for the rig if I use the MSI 890FXA-GD70 Mobo.

I'm building a new rig and plan on purchasing a less expense Mobo this time around.
I heard the MSI mATX h61-p21 @ $58 bucks is a good 4x Mobo. That's $200 less than the Gigabyte but i'm afraid it'll have the same performance issue that I'm experiencing with the MSI 890FXA-GD70. I'd much rather spend the extra $200 and have 154 mhash better performance.

The question is should I expect different Mobo's to have different performance? If so what Motherboards perform the best?

Thanks for your help guys!

[WhitePhantom]

I wonder if it's caused by the same issue I had when I swapped 5970s in a computer and lost a good chunk of my hashing power.  My last post in this thread points to the info that fixed my issue.

https://bitcointalk.org/index.php?topic=62925

Good luck!