interdum

Interdum is inter plus dum, "in the meanwhile", and it means "sometimes" in the sense of "at intervals": the action recurs, with gaps. It normally goes with the present or the imperfect, the tenses of habit.

The nearest rival is aliquando, and the difference is worth keeping. Aliquando means "at some time or other", possibly once, and points at an indefinite occasion; interdum means "from time to time", and implies repetition. Latin loves interdum in balanced pairs, interdum ... interdum, "sometimes this, sometimes that", where English would say "now ... now". Do not confuse it with interea, "meanwhile", which sets one action against another rather than describing frequency; the shared inter- makes them look closer than they are.