Wow those are pretty frequent stutters. Your computer specs look fine and should handle GTA no problem - other than your RAM which is borderline. I usually hit 7.2 GB RAM usage - nonetheless, you should still be able to play without stutters or major issues like this.
Here are a few questions:
1. Have you tried turning off the Reflection MSAA in your GTA graphics settings?
2. Have you tried reducing Distance Scaling, Reflection Quality, Shadow Quality in your GTA graphics settings?
3. Have you tried turning off the Radeon FPS cap?
4. Are you using the latest Readeon driver?
5. Is your CPU maxing out (100%)?
That would be my first line of attach there. Hopefully, something works there!