There is a reset button the overdrive section which would reset things back to the defaults, as well as the reset to factory defaults option in the preferences section of Radeon Settings.
Yes, I did that too before. But now I realized when I underclock my default max memory from 1400 Mhz to 1300 Mhz , gpu is stable and there is no artefacts at all. However 1400 Mhz for Memory clock and anything above is unstable. GPU clock is not problem. But isn't it strange that I have to underclock my core clocks? So, for me now there are two options:
1) Memory clock to be stuck on max (even in idle) and not fluctuate at all - no flickering
2) Underclock my core Memory clock by 100 Mhz - no flickering
This is really weird and it happens only when Memory clock fluctuates from min to max, but on CORE CLOCK speeds (GPU clock - 955 Mhz, MEMORY clock - 1400 Mhz or above up to 1500 Mhz). I tried all drivers, all settings, but nothing helped. I have this problem for almost 1 year and still there is no solution. Someone told me that low core voltage can cause flickering and that voltage can be modified via some 3rd party applications.