John Farmer & William Henley, Slang & Its Analogues (1904) offers this saying to describe a person who tends to be very pleasant or very unpleasant:
He is all honey, or all turd.
A much older version of the expression appears in Francis Grose, A Classical Dictionary of the Vulgar Tongue (1788), describing a relationship that alternates between extremes of lovey-dovey bliss and unrepressed rage was
It is all honey or all t——d with them ; said of persons who are either in the extremity of friendship or enmity, either kissing or fighting.
On the single-word front, one option is oscillator, which Merriam-Webster's Eleventh Collegiate Dictionary (2003) conveniently defines as "one that oscillates." The Eleventh Collegiate does better in offering a relevant definition of oscillate:
oscillate vi (1726) ... 2 : to vary between opposing beliefs, feelings, or theories
and it offers these relevant definitions of oscillation:
oscillation n (1658) ... 1 : the action or state of oscillating 2 : VARIATION, FLUCTUATION ... 4 : a single swing (as of an oscillating body) from one extreme limit to the other