Sometimes overclocked (too much), overheats and mod bios can cause the artifacts ... I don't know what methods did you used due to increasing the hashrate, but you should do it step by step (don't do all the thing at the same time).

Please give more info

- which AMD driver did you installed?
- What kind of Bios did you flashed .. One click timing or you just download mod bios from any site?
- Did you check GPU memory error at HWinfo?
- etc