Irregular or unpredictable (7)
I believe the answer is:
erratic
I believe this is a double definition.
'irregular' is the first definition.
(I've seen this before)
'unpredictable' is the second definition.
(synonyms)
(Other definitions for erratic that I've seen before include "Irregular in pattern or movement" , "Wild" , "on irregular path" , "Shifting like Cartier" , "Unpredictable - car tire (anag)" .)