Your GPU is clocking its own speed down trying to keep itself cool. Grab HWMonitor and confirm the temperatures of that and the rest of your computers and work out a cooling solution
I'm mining with Ubuntu and atitweak and aticonfig tell the same.
I've flashed the gpu bios from version 015.012.000.004.000000 (113-AD44400-101) to 015.039.000.000.000000 (113-AD47700-100) but now the gpu doesn't boot with this bios version. Maybe the 280X bios works.
Does someone have a working link to Stilt bios versions?