Initially thought this was limited to GTA V but appears to be affecting other games (such as Dying Light)
I've run through just about every configurable setting, downloaded and reinstalled (after an uninstall/reboot) and messed about with every recording format in Mirillis action. I can't get it to work properly.
1. GTA V won't launch if Mirillis Action is already running. You must run mirillis afterwards. - work around is simply to launch Mirillis after in-game.
2. Frame rate plummets when recording active, I don't get this sort of issue with Shadowplay and I record to the same device (and Shadowplay files are larger).
3. Sony Vegas shows no video track but two audio tracks on recording, strangely I can play the files and they work fine with decent quality video showing. This is probably the strangest one, other videos recorded using other products show up fine.
OS: Windows 8.1
Video Card: Inno 3D Airboss Ultra 980
Memory: 16 GB
Monitor Resolution: 1920x1080 @ 120Hz (usually record at 60)
Video Editor: Vegas 13
Game Problem: GTA V, Dying Light plus others.
Video Type: AVI and MP4 (native and Nvidia NVENC). MP4/MP4 NVENC playback OK but have a huge FPS hit, AVI plays back with a bit of hitching/stuttering video but lower FPS hit.
Additional: Tested with multiple SSDs and mechanical HDDs, HDD activity doesn't seem to be high (Aida 64 monitoring) whilst recording and this affects both short <5s clips and longer ones.