1) A once while 1) Anon 2) At times 3) Betimes 4) Erratically 5) From time to time 6) Now and again 7) Now and then 8) Off and on 9) On and off 10) Once in a blue moon 11) Once in a while 12) Rarely 13) Sometimes 14) Sporadically 1) At times 2) From time to time 3) Now and again 4) Now and then 5) On occasion 6) Once in a while 
• (adv.) In an occasional manner; on occasion; at times, as convenience requires or opportunity offers; not regularly.
Oc·ca'sion·al·ly adverb In an occasional manner; on occasion; at times, as convenience requires or opportunity offers; not regularly. Stewart. « The one, Wolsey, directly his subject by birth; the other, his subject occasionally by his preferment.» Fuller. 
[adv] - now and then or here and there
sometimes
on occasion adverb now and then or here and there; `he was arrogant and occasionally callous`; `open areas are only occasionally interrupted by clumps of trees`; `they visit New York on occasion`; `now and again she would take her favorite book from ...