Hi Klinge. Can you share information about your setup and can you let us know if all previous drivers have been thoroughly sweeped from your system before installing the HD 7870 Joker?
By using the DDU application you can sweep all old driver versions: DDU app
Then proceed to install the brand new AMD Catalyst 15.7 Driver from here: http://support.amd.com/en-us/download?
If this process does not solve the issue we need to look for a different solution.