I would understand "sometime" as "at one point in time that I am being non-specific about", and "sometimes" as "at various points in time; occasionally".
One is a one-time thing, the other is a repeated thing.
Whereabouts do you live that they are used interchangeably?
(Or perhaps I should ask: when do you live? I see that one dictionary says that "sometime" may have the meaning "sometimes" but marks this as "Archaic". Not a usage I had encountered before but perhaps I'm too young.)