ahhhh same problem -_-
i think it's only mine, currently im use saphire Rx 480 (ref) + XFX rx 480 black oc (reff) , im confused the core of XFX Rx 480 down from 1135 (in first running) to around 800mhz after 30minute, and the hashrate down to 24-26 too , but the saphire Rx 480 runing smoothly and very stable at 30.5mhs

any solution for this case ?