The phrase "on a roller coaster" means "坐过山车“ or "情况的起伏”，my sentences are following:
1. He is always <font style="background-color: yellow;">on an emotional roller coaster</font>. Maybe now he is happy, but next second he is angry.
2. Before college entance examination, his scores of practice examinatioans are<font style="background-color: yellow;"> on a roller coaster</font>, which makes his parents worried.
Thank you!
The first one is good (but check your tenses).
The second one would never be said by a native speaker though the meaning is clear enough. I think it doesn't sound good because you are talking about figures, not something more active and dynamic, like a person. We might say:
e.g. His scores are up and down/unpredictable/all over the place
