To swing back and forth with a steady, uninterrupted rhythm.
To waver, as between conflicting opinions or courses of action; vacillate: "The court has oscillated over the decades from more liberal to less, more conservative to less, depending upon who was president at the time of vacancies” ( Gordon J. Humphrey). See Synonyms at swing.
Physics To vary between alternate extremes, usually within a definable period of time.