No, because I don't care about averages. I care about minimum, and maybe maximum.

The worst for representative democracy is literally Hitler. The second worst (you could argue that Hitler was a unique set of circumstances) is the dysfunctional German, Italian, or French states.

The best case is probably Switzerland (party proportions barely change, I assume because the power of parliament is diminished because of elements of direct democracy that Swiss practice).

What's the worst case for first-past-the-post democracy?

Thanks for being transparent. When I say stable, I'm referring to regime-years which is a standard way of operationalizing stability in comparative politics. It makes sense that if you're interested in things other than regime-years then most of what I have to share isn't going to be interesting to you.