Hey there,
Thanks for the details. If you're able to play some games at 120, having that randomly dropping to 15 would definitely be pretty disruptive. Since this doesn't sound super consistent, there are a few things to check so we can narrow down what could be causing that.
Close any background applications that might interact with your games (Discord overlay, Game Bar, Overwolf, MSI Afterburner/RTSS, RGB software, etc.). If that doesn't help, the next step would be trying a clean boot to rule out another background service interfering with the game.
If you're using AMD features such as Radeon Anti-Lag, Radeon Chill, or an FPS limiter, try disabling them temporarily and test again. If you're still running into this after that point, could you let us know which AMD GPU you're using? When the FPS drops, does that start immediately when a match begins, or just after playing for a while?