run hot and cold

run hot and cold

To vacillate between two opposing or starkly different states, opinions, or behaviors. Dan: "So, how are things going between you and Mallory?" Sarah: "Hard to tell. She runs hot and cold one day to the next, so I can never tell how she really feels!" The boss has been running hot and cold about whether or not we're going through with this project. I wish she would just make a decision!
See also: and, cold, hot, run