Not particularly satisfied with 'occur' as an alternative to 'go on'. I'd think of it as 'happen', 'transpire', or some other transitory event rather than a continuing process. My interactive OED doesn't suggest 'go on' as an alternative to 'occur' either.
I don't see Saltyolddog's problem with the clue?
He has already said that he thinks "go on" is synonymous with "happen"...
"Occur" and "happen" mean the same thing.
See Bradford's dictionary