About mafia III crashing I think its pretty normal, the game is pure GARBAGE and a bug fest festival.
Now the pixel error how do they look?
If its like this (doesnt look as bad since thats a zoomed screenshot), its normal and its a known AMD problem for years from what I read on forums but AMD dont care and never added that to their known problem list to be fixed...
1- If its like this or other artifact problems and happens on lot of games, then use DDU app in safe mode and if you had a Nvidia previously and never used DDU, clean Nvidia drivers first, then restart and open DDU again in safe mode and clean AMD drivers, then download latest AMD drivers and see if the problem persists.
2- Reinstall VC++ and DirectX
3- use only one monitor at a time and try with a different cable.
If the problem persists continue:
4- check if the GPU is running at stock reference clocks for that GPU, if its higher downclock to the reference values and do a FurMark Burn-in for a few hours and keep checking if anything strange appears, like black lines\dots.
if the problem get fixed, someone might have flashed a bios with OC, check the Vbios and try to flash it with the latest official Vbios for your GPU model.
If the problem still happens I'm almost sure its faulty and its usually memory or the central GPU that have problems and might also be that someone pushed too much clocks and damaged the GPU.