Unstable seems to be in common use, but I have not often run across instable. In mathematics, engineering, electronics, and aeronautics I have often seen and used astable. As in "A rocket …

2011年4月6日 · Nouns in "-ity" don't always take the same negative prefix as the corresponding adjective. For example, "instability" is greatly preferred over "unstability", but "unstable" is …