erratic, fickle, mercurial, quicksilver -- (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")
erratic, planetary, wandering -- (having no fixed course; "an erratic comet"; "his life followed a wandering course"; "a planetary vagabond")
erratic, temperamental -- (likely to perform unpredictably; "erratic winds are the bane of a sailor"; "a temperamental motor; sometimes it would start and sometimes it wouldn't"; "that beautiful but temperamental instrument the flute"- Osbert Lancaster)
(Sound)
