Do you have any kind of antivirus?
Are you on Windows?
I know you probably did that already, but I would still mention the following: try using DDU (display driver uninstaller) or AMD driver removal utility from safe mode, disable your antivirus/encryption software if there is any, then install driver. Also make sure no slecific software is running in background during installation - like Afterburner, GeDoSaTo, RTSS or some other applications that may or may not be intrusive. For safety, just close all your tray/autorun apps you have before starting to install.
If your motherboard has multiple PCI-E slots (they usually do, the second one is just 8x... unless it's a super fancy overtop mobo, then it'll also be 16x) try putting your GPU there and see if it fixes anything. Try using different connection port (e.g. HDMI or another DisplayPort), although it wont help, probably.
If all else fails, try putting the GPU into a different PC and see if the issue repeats itself.
