Sometime refers to a single action in the future? It does refer to a single point in the future, yes, although of course it's possible that the action might be repeated. If you said "Do you fancy a game of tennis sometime?" then that refers to one single game, but of course it may lead to playing more often.
Sometimes more like on a regular basis? Yes. Sometimes is a description of regularity, like "always", "never", "often", "occasionally" etc.