I can not spell this out accurately enough to file a bug, but when I have been editing a clip (watching, listening, adding guides, playing and replaying segments) after a half hour or more, the video rendition in the project monitor sometimes begins to stutter more and more, and placement of the active point on the timeline becomes increasingly difficult.

Saving and restarting Kdenlive restores normal operation.

Can you tell us something about memory and CPU usage? (how it develops)