Let's look at "instability" from a user's point of view.
From day to day or even from minute to minute, Google's order of results for [keyphrase] may vary slightly. Yesterday, John Doe's page was in position 4, and Jane Buck's page was in position 5. Today, their positions are reversed. Does that mean Google's search results are "unstable?" Even if it does, how likely am I to notice, unless I'm a site owner who's checking to see where I rank?
The only time "instability" is likely to matter (from a user's perspective) is when an expected result is nowhere to be found. Knock a Wikipedia or Amazon result from the top 10, and large numbers of people will be irritated. Banish Nick Nobody's result from the top 10, and nobody but Nick is likely to know or care.