Rx 580 help

I recently got sapphire RX 580 nitro + that hashes at 22.5mh/s and whenever i seem to overclock it at all, it drops hash rate to 18-20mh/s even when i modded the bios the hash rate stays at 22.5 mh/s?

using windows 7 on 17.5.3 drivers

any help would be great!