By turn - Idioms by The Free Dictionary
In an alternating cycle; in successive or rotating turns. I think something was wrong, for her manner was by turns welcoming and cold, which made for a disconcerting evening. My wife and I divide household tasks by turns.
by ˈturn(s) used when talking of contrasting feelings or actions which follow each other: He looked surprised, worried and angry by turn. ♢ When they told me I had got the job in New York, I felt by turns excited and anxious.