Synonyms for Once in a while:

a. & adv.

once in awhile (adjective, adverb)
occasionally, now and then.

adv.

once in a while (adverb)
at times, from time to time, seldom, now and again, on occasion.

adv

every now and then (adverb)
occasionally.
infrequently (adverb)
seldom.
now and then (adverb)
at times, now and again, from time to time, on occasion.
once in a while (adverb)
now and then.

n.

seldom. once in a while (noun)
at times, now and then, occasionally, now and again, on occasion, from time to time.

Other synonyms:

from time to time
from time to time.
rarely
seldom.
Other relevant words:
seldom, occasionally.