erratic
/ɪˈrætɪk; ɪ`rætɪk/ adj
(usu derog 通常作贬义) irregular or uneven in movement, quality or behaviour; unreliable （动作、 品质或行为）不规则的, 无常的, 不可靠的
Deliveries of goods are erratic. 不按规定送货.
The singer gave an erratic performance. 演唱者未唱出水平.
This clock is rather erratic. 这个钟不太准.