como menciono @Ariane, "intermitente" es la palabra que buscas it is the same word that is used when your cable or electricity is not continuos,it comes and goes... on-and-off 'He estudiado español de manera intermitente desde hace algunos años' you can also use the word "discontinua" , but it sounds too elegant and it is really uncommon, but it is completely correct 'He estudiado español de manera discontinua desde hace algunos años' Saludos

Se me ocurre "a intervalos" o "de forma intermitente."

Hello, there's no exactly translation for the expression "on and off" in Spanish, the best thing you could do is saying "I started studying Spanish many years ago, but not continuously", "He empezado a estudiar el español hace muchos años, pero no estudié continuamente" or something like that, I believe.

In my opinion it would be easier to express this as 'from time to time' .... de vez en cuando

on: encendido off: apagado