Great, we've ruled out a possible error-correction explosion effect.
Curvey and zigzaggy framerates are okay, it's those massive spikes (1% and 0.1%) that are the more visible stutter problems. What I need to know is your frame rate pattern. Are you able to post any RTSS frametime screenshots?
Test #1
1. Install RTSS if you haven't yet
2. Turn on RTSS
3. Click "Show On Screen Display"
4. Click Settings button
5. Select "Enable Frametime History Overlay"
6. Click OK
7. Now play your game and watch your framerate oscillations.
There are dozens of different kind of framerate instabilities, and seeing the shape can help diagnose a speedier resolution to the stutter problem, or at least isolate the cause better, from drivers to game inefficiencies, etc.
Test #2
Also try capping your framerate to the middle of your game's framerate range, and see what happens to your stutters.