Every so often (4,2,1,5)
(Another definition for on occasion that I've seen is " Sometimes".)
(Other definitions for once in a while that I've seen before include "Every now and again" , "At times" , "Occasionally" , "Not very often" .)
(Other definitions for off and on that I've seen before include "Now and then, stop and start" , "Intermittently" , "Intermittent" , "switch positions" , "the two sides at the Oval are this" .)