As far as I can tell, the differences involve number:
Iam means once, or sometime; at some unspecified (single) time.
Foje can mean once or sometimes; it's the adverbial form of fojo which means occasion or (a) time, so you can thing of foje as meaning "at a time" or "occasionally"
Kelka means some, so kelkfoje (or kelkafoje, though I see it's frowned on for an excess of syllables) means sometimes or occasionally. :)
Surfing round, I see kelkfoje used as "now and then," "now and again."
All are adverbs.