Is it permissible to pray behind a husband who leads the prayer sometimes and abandons it at other times?

If the husband's prayer is valid and meets all conditions and pillars, then your prayer behind him is valid. For whoever's prayer is valid for himself, his imamate (leading of prayer) for others is also valid. As for the abandonment of some prayers, it does not affect the validity of what he has prayed. Regarding the imamate of a fasiq (transgressor), despite the weakness of the hadiths mentioned concerning the validity of prayer behind every righteous and wicked person, and the conflicting hadith (definition) "Let not one who is audacious in his religion lead you in prayer," the rule is that whoever's prayer is valid, his imamate is also valid.