I would pick from yhe following list: a) in modo incostante b) senza costanza ( = /non con costanza/ but direct meaning) c) saltuariamente d) in modo episodico e) quando posso There are different shades of meanings, if not clear, please ask again.

"a fasi/periodi alterni" "un po' si e un po' no" "incostantemente" "non regolarmente" "ogni tanto" There is no general way to convey the meaning of "off and on", the best way is to build a periphrasis using the expressions above, or the ones suggested by the other users.

I think "ogni tanto" is the most common way to say this.

Ogni tanto is the most used.

In addition to the answers you have already had I say: "ad intermittenza". For instance: Studio l'italiano ad intermittenza.