Google rankings are stable
This discussion is faulty because the underlying assumption that the rankings are unstable is incorrect. The fact is that 99% of rankings are unchanged during most algorithmic changes. Go back and read the announcements. Most of the algo announcments state the changes affect single digit percentage of searches.
There are problems but not instability
I'm not saying Google's SERPs are the apex of search. Even Googlers are aware that improvements are necessary. If anything, my personal opinion is there are problems with Google's search results, but instability of rankings is not one of them.