erratic

Adjective

• likely to perform unpredictably; sometimes it would start and sometimes it wouldn't"
  • erratic winds are the bane of a sailor
    
  • a temperamental motor
    
  • that beautiful but temperamental instrument the flute- Osbert Lancaster
    

• having no fixed course
  • an erratic comet
    
  • his life followed a wandering course
    
  • a planetary vagabond
    

• liable to sudden unpredictable change
  • erratic behavior
    
  • fickle weather
    
  • mercurial twists of temperament
    
  • a quicksilver character, cool and willful at one moment, utterly fragile the next
    

Related words

• Synonyms: 
  temperamental , planetary , wandering , fickle , mercurial , quicksilver(a)