In general, intransitive verbs can not be used in the passive voice, with one exception. When a prepositional phrase follows, it is possible
He looked at me <-> I was looked at by him
She kept on talking (The verb kept on is intransitive in this sentence because it has no object.)
keep on
intransitive verb inseparable
1. [continue] continuer
they kept on talking ils ont continué à parler
I keep on making the same mistakes je fais toujours les mêmes erreurs
http://www.larousse.fr/dictionnaires...is/keep/590582