Impossible to predict

• Off and on
• Mercurial
• Not at all consistent
• Unusually rare twitching, lacking predictability
• Not fixed
• Hit or miss

Douglas Harper's Etymology Dictionary Word definitions in Douglas Harper's Etymology Dictionary
late 14c., "wandering, moving," from Old French erratique "wandering, vagrant" (13c.) and directly from Latin erraticus "wandering, straying, roving," from erratum "an error, mistake, fault," past participle of errare "to wander, err" (see err ). Sense ...

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
Erratic \Er*rat"ic\, a. [L. erraticus, fr. errare to wander: cf. F. erratique. See Err .] Having no certain course; roving about without a fixed destination; wandering; moving; -- hence, applied to the planets as distinguished from the fixed stars. The ...

Wikipedia Word definitions in Wikipedia
Erratic may refer to: Erratic, a project of music artist Jan Robbe Glacial erratic , a piece of rock that differs from the size and type of rock native to the area in which it rests Erratic ant ( Tapinoma erraticum ), a species of ant