speed decrease when Mining in parallel mode(ZEC) (3x R9 280x)

[_AnT_]

hI,
I have the following problem: I have 3 video cards, two  XFX R9 280X and one Sapphire R9 280X OC.
I'm mining on ZEC. When I have all three video cards running on Clymore MIner, the result of two XFX R9 280X is 42 H/s & 43 H/s
And the result of Sapphire R9 280X OC is 32 H/s (to be considered that this model is better than two others)
When i'm running only  Sapphire R9 280X OC - the result is about 45 H/s .
What is the problem? Why does the speed decrease when running in parallel mode? How can I settle this?

[sirslayer]

maybe its NUMA settings in your bios??  google NUMA memory nodes

[_AnT_]

Bios of VGA or MB ?

[toptek]

Don't know if this helps but if you use after burner see if boost is on in after burner . not sure if 280x have that my 280 not X does and having it on lower speeds on mine but it's only one card  and  280 not the X and disable ULPS


my 280 not X

 one card with boost off and disabled ULPS . no over clocking.

ZEC - Total Speed: 47.000 H/s, Total Shares: 3174, Rejected: 17, Time: 09:31