erratic

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”

likely to perform unpredictably

• “erratic winds are the bane of a sailor”
• “a temperamental motor; sometimes it would start and sometimes it wouldn”
• “;”