Possible Clues:

• Periodic
• Occurring infrequently
• Happening now and then
• Random 'Notes on a Small Island' seen in Circle Line
• Herbert's verses, from time to time
• Periodical table may be of such a type
• Table not used every day?
• Give rise to chap - that's not very common
• Casual and cool as I can, possibly
• Episodic