Definition of flare ups:
(v) :
(intransitive) To become more intense suddenly.
(v) :
(intransitive, sometimes with “out” instead of “up”) To burst out suddenly, as in anger.
(v) :
(intransitive) To burn brightly again.
(n) :
Alternative form of flare-up. [A sudden outbreak, outburst or eruption (originally of flame, but now used more generally of any violent activity or emotion).]