erratic vs unstable

erratic

adj
  • Unsteady, random; prone to unexpected changes; not consistent. 

  • Deviating from normal opinions or actions; eccentric; odd. 

noun
  • A rock moved from one location to another, usually by a glacier. 

  • Anything that has erratic characteristics. 

unstable

adj
  • Unpredictable. 

  • Readily decomposable. 

  • Not stable. 

  • Radioactive, especially with a short half-life. 

  • Fickle. 

  • Having a strong tendency to change. 

  • Fluctuating; not constant. 

verb
  • To release (an animal) from a stable. 

How often have the words erratic and unstable occurred in a corpus of books? (source: Google Ngram Viewer )