De temps en temps

De temps à autre

These two have the same meaning, it means ‘from time to time’.
The latter is slightly more formal and could indicate a slightly lesser frequency.

De façon intermittente

It means directly ‘intermittently’; it’s more precise than the two above.

If you want say something like switch on or switch off you must say :

Mettre quelque-chose en marche ou à l’arrêt.

You can’t say :

Mettre quelque-chose en marche ET à l’arrêt.

because it’s one or the other but not the two together.

It depends on the time pattern: French and English don’t break them up in exactly the same way.

For a habitual action like “I play tennis on and off”, you could say “Je joue au tennis à certaines périodes”; this has no implication on which of the on and off periods are the longer ones. Some other possibilities tend imply shorter on periods: “de manière sporadique”, “sporadiquement”, “de façon intermittente”, “par intermittence”. As for “de temps à autre” or “de temps en temps” or “à l’occasion”, they can mean “now and then” or “once in a while” (i.e. there are no lasting on periods, just the occasional game) as well as “on and off”.

For an interrupted activity like “I knit on and off, when I’m not helping a customer”, you can say “Je tricote par moments, quand je ne suis pas occupé avec un client”.