Erratic
WordNet
adjective
(1) 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"
(2) 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
(3) Having no fixed course
"An erratic comet"
"His life followed a wandering course"
"A planetary vagabond"