undependable

/ˌʌndɪˈpɛndəbəl/
adjective
  1. Not able to be trusted or relied on; not consistent in performance or behavior.
    • She stopped hiring him because he was undependable and showed up late to every job.
    • The weather in spring can be undependable, changing from sunny to rainy in minutes.
    • My old car is undependable and often breaks down on the highway.