Synonyms for Every once in a while:

• adj.

  •periodic (adjective)
  • periodical,
  • Isochronal,
  • intermittent,
  • cyclic,
  • cyclical,
  • Recurring,
  • alternate,
  • sporadic,
  • regular,
  • perennial,
  • daily,
  • rhythmic,
  • infrequent,
  • repeated,
  • annual,
  • spasmodic,
  • routine,
  • recurrent,
  • yearly,
  • Isochronous,
  • serial.

• adv.

  •every now and then (adverb)
  • ever so often.
  •from time to time (adverb)
  • from time time.

• Other synonyms:

  •unexpectedly
  • haphazardly.