Kodi Leia 18.3 for RPi4 with Raspbian Buster plays well , videos not choppy, audio (although HDMI and Analog only, DAC HATs not supported) also in sync with video.
gpu memory set at 264
Kodi Kypton 17.6 in Raspbian Buster (RPi3B+ and earlier models), user interface choppy, and other symptoms as presented by OP.
Installing Kodi Leia 18.3 in Raspbian Buster for RPi3B+ and earlier models does not run, its built specifically for RPi4B only!
Kodi Leia 18.2 on Raspbian Stretch is the only option for RPi3B+ and earlier models...
"Don't come to me with 'issues' for I don't know how to deal with those
Come to me with 'problems' and I'll help you find solutions"
Some people be like:
"Help me! Am drowning! But dont you dare touch me nor come near me!"