Just some basic ideas to start with…
Which rendering backend is in use? (You might try changing to see whether this helps with stability)
qdbus org.kde.KWin /Compositor org.kde.kwin.Compositing.compositingType
Is desktop effects active?
qdbus org.kde.KWin /Compositor org.kde.kwin.Compositing.active
If enabled, have you tried turning them off by toggling with Shift-Alt-F12?