Maybe the solutions I have used can serve as a reference to make changes more adjusted to your preferences and your type of TV. I also had some problems with the video output (distortions and image instability and color distortions). I solved this by forcing a single resolution, without the possibility of selecting another one, from the start of my Mecool KIII Pro (S912) and modifying the Kodi system settings.
File /flash/config.ini:
hdmimode=‘720p50hz’ (Added 09172019: I have seen that this does not work and I no longer use it)
File /storage/.kodi/userdata/disp_cap:
720p50hz (does not allow another resolution)
File /storage/.config/autostart.sh
fw_setenv hdmimode 720p50hz (fixed startup resolution)
fw_setenv outputmode 720p50hz (fixed startup resolution)
Settings in Kodi->System->CoreELEC
Disable noise reduction: ON
Disable deinterlacing: ON
Limit display color depth to 8-bits: OFF (solves color distortion)
Use 4: 2: 2 color subsampling: OFF (solves color distortion)