Logs were failing while in fullscreen mode. But I was abel to grab the frame time stutters in the Performance::Advisor section of Radeon Software: Adrenaline edition.
Steps to reproduce: Start LOL in fullscreen mode. Alt tab to second monitor. Alt tab back. After playing for 1 - 5 minutes large stutters appear. Frame times up to 60ms.
Fix: Go to windowed, then back to fullscreen in LOL. Stuttering disappears until next alt tab.
Second resolution. Forcing GfxClock > 1200 through game profile eliminated stutter.