How big is your PSU, perhaps it's not delivering enough amperes to the card, and the card notices?

Don't think.....PSU are 1000 for 7970 and 1050 for 280x

I don't know about these two:
"lookup-gap" : "2,2,2",
"gpu-dyninterval" : "7",

About these value i dind't find a lot on google but there is some info in the GPU-README of cgminer.
Looks like that if u set "thread-concurrency"  you don't need to set "shaders" , "lookup-gap" , and "worksize" ( if I remember correctly).
Some value like "gpu-dyninterval" are there because when I found a good setting i print it with cgminer and it add a lot of value i don't understand.

ah, nice to know.
What did you use to edit the r9-280x bios?  Also VBE7 ?

I changed State #0 and State #6 Engine,MEM,Voltage:
Engine: 930
Mem: 990
Voltage 0.980

but the voltage was to low ( i had no time to test it too mutch) so i had to set it higher with cgminer ( 1020 looks ok , but for 1 card was not enought)

> I builded cgminer my self with AMD-APP-SDK-v2.9-lnx64.tgz ,
could that also be a reason for the good performance?

Don't think because pre-builded cgminer is using an older version I think ( not sure wich one)


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Later I will try your value ( realy low voltage) .... going down with MEM under Engine (never tried).