Instability is a synonym of fickleness.
As nouns the difference between instability and fickleness
is that instability
is (uncountable) the quality of being unstable while fickleness
is the quality of being fickle.
(uncountable) The quality of being unstable.
(physics, countable) A state that is not in equilibrium, or in which a small change has a large irreversible effect.
The quality of being fickle.