low hash rate on 480 nitro OC 8gb - tips appreciated

hi,
few days ago i bought 2 new 480's (now i have 4 in total).

my rig config is:

asrock 97 anniversary
cpu g3460
win 10
16gb ram
ssd 128gb
psu 1200w
usb raiser v005

amd driver 16.9.2

Jukebox mod rom v4 from here https://forum.ethereum.org/discussion/9650/sapphire-rx-480-nitro-oc-8gb-11260-01-20g-modded-bios-29-mh-downvolt/p5

3 card safely reach 29.3 mh/s at 1200 clock / 2150 mem
1 card (one of those new) can not reach more than 24.1 mh/s at 1000 clock / 1800 mem.



if i try to up with the clock or mem it crashes.

any ideas or suggestions ?

thank you very much
A.