I believe bi-polar disorder can show itself with the sufferer having periods of extremely irrational behaviour, mixed with periods where they seem to behave "normally".
It really depends on what you mean by "insanity". There are plenty of people who behave in an insane manner some of the time, but it's just part of their personality and could simply be called eccentric.
I can't think of a word that would describe an actual disease, but would also be used for someone who is simply quick-tempered.
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