What is the ruling on intermittent bleeding that recurs at varying intervals, sometimes followed by a brownish discharge, approximately a year and a half after the end of postpartum bleeding?

As long as it is blood followed by yellowish and brownish discharge and it continued for 9 days, it is considered menstruation (hayd). Likewise, what you saw after 19 days is menstruation, and the third time is also menstruation; because the minimum period of purity (tuhr) between two menstruations is 13 days. As for the brownish discharge the fourth time, 10 days after the cessation of blood, it is not considered menstruation; because its total with what preceded it exceeds 15 days, and there were not 13 days separating them.