1. Since we are troubleshooting reset wattman and let it run at it's default settings.
2. Type Exploit in the search box. Click on Exploit Protection. Go to Program Settings. Add Mafia2.exe and Overide every option by setting them OFF.
3. Uncap the frame rates in Mafia 2.
4. Download CRU:
https://www.monitortests.com/forum/Thread-Custom-Resolution-Utility-CRU
And follow the instructions on this pic (not mind).
-open CRU
-Click on the option under Extenstion Blocks and select edit
-Locate were your freesync range is and click on it
-Once highlighted blue use the arrow to move it up to the top
-Click OK
That's it.
Once you do that try Mafia 2 again.
Post back your results.
Edit:
Make sure ISLC is still running correctly in the background.