occasionally, on occasion, once in a while, now and then, now and again, at times, from time to time(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 the shelf and read to us"; "as we drove along, the beautiful scenery now and then attracted his attention"

on occasion(Adverb)

occasionally, from time to time, now and then