波があるって英語でなんて言うの？

Here are two ways that we can express our motivation levels with the most common expression being about how the motivation, "comes in waves," in a way to say that in falls and rises similar to how waves in the ocean move. We can also say that you have, "good and bad days," in regards to your motivation.

ある時はやる気がでるけど、ある時はでない。
やる気＝motivation、やる気がある＝feel motivated, feel determined 
Sometimes~, sometimes~. ＝～な時もあれば、～な時もある。よく使われています！

訳:私のパフォーマンスは気分によります。
depend on は非常に汎用性がたかいです。「〜による。」

「波がある」のは「やる気」だと今回は捕らえました。
やる気、つまりモチベーションが安定しないから、波があるのだろうと思います。
そして「波がある」というのはつまり安定がない、不安定であるということと同義です。
すなわち、my motivation is unstableという文章が一番しっくりくるのではないでしょうか。

If you wanted to explain that sometimes you are highly motivated and other times you are not, you could say either of the following to express this: "I have good days and bad days with my motivation" or "My motivation can be really up and down, it is not always the same".

Examples:

I have days when I feel highly motivated, I also have days where I don't feel very motivated.
I have a lot of ups and downs when it comes to motivation.
My motivation levels fluctuate frequently.

1. I often vacillate between feelings of high motivation and
   feelings of laziness and indifference.
   To vacillate means to waver between two different actions
   or opinions. Indifference means having no interest.
   Example: I am vacillating between dentistry and optometry.

2. One day I feel highly motivated and the next I seem to experience a
   slump.
   To be highly motivated means having a strong desire to succeed at
   a project or a task. When you experience a slump then it means
   that you are apathetic or tired and don't really care one way or the other.
   The next day you go back to tackling the task at hand until tiredness
   or irritation overcomes you again. Apathetic means being disinterested.