In AMD WATTMAN in Radeon Settings, set the Power level to 50% and see if the artifacts occurs.
If you still get artifacts, then try stress testing your GPU card and see if those artifacts appear.
Artifacts can be caused by Overheating GPU card. So when stress testing the GPU card check out the temperatures and PSU voltages. See if anything is abnormal. Also make sure all the power PSU GPU cables are connected to the GPU card.
I personally like using OCCT, stand alone program and free, to stress test my CPU, GPU, and PSU. Maybe also Furmark to check for cooling issues or GPU stability issues.
