We have an inordinate number of synonyms with this meaning: sometimes occasionally now and then every now and then once in a while every once in a while every so often from time to time now and again every now and again at times on and off periodically The only formal ones are occasionally and periodically. Sometimes is probably the most neutral in register.
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samerrustomI will be glad if you give one example for each one of the above synonyms.You can put them all into the same sentence and they will be natural: