WordNet (r) 3.0 (2006):

unpredictable
    adj 1: not capable of being foretold [ant: predictable]
    2: unknown in advance; "an unpredictable (or indeterminable)
       future"
    3: not occurring at expected times [syn: irregular,
       unpredictable]

Moby Thesaurus II by Grady Ward, 1.0:

148 Moby Thesaurus words for "unpredictable":
   accidental, adrift, adventitious, afloat, agnostic, aleatory,
   alternating, ambiguous, amorphous, capricious, casual, causeless,
   chance, chancy, changeable, changeful, contingent, coquettish,
   destinal, desultory, deviable, dicey, dizzy, doubtful, doubting,
   dubious, eccentric, equivocal, erratic, extraordinary, faddish,
   fast and loose, fatal, fatidic, fickle, fitful, flickering,
   flighty, flirtatious, flitting, fluctuant, fluctuating, fluky,
   fortuitous, freakish, giddy, hesitant, hesitating, iffy, impetuous,
   improbable, impulsive, incalculable, incidental, inconsistent,
   inconstant, indecisive, indemonstrable, indeterminate, infirm,
   insecure, irregular, irresolute, irresponsible, light, mazy,
   mercurial, moody, more than expected, out-of-the-way,
   past expectation, polysemous, quicksilver, rambling, restless,
   risky, rocky, roving, scatterbrained, shaky, shapeless, shifting,
   shifty, shuffling, skeptical, skittish, spasmodic, spineless,
   sudden, tottery, touch-and-go, toying, treacherous, unaccountable,
   unannounced, unanticipated, uncaused, uncertain, unconfirmable,
   uncontrolled, unconvinced, undependable, undetermined,
   undisciplined, undivinable, undivined, unexpected, unfixed,
   unforeseeable, unforeseen, unguessed, unhealthy, unheralded,
   unhoped for, unlooked-for, unpersuaded, unpredicted,
   unprepared for, unprovable, unreliable, unrestrained, unsafe,
   unsettled, unsound, unstable, unstable as water, unstaid,
   unsteadfast, unsteady, unsure, untrustworthy, unverifiable,
   vacillating, vagrant, variable, versatile, vicissitudinary,
   vicissitudinous, volatile, wandering, wanton, wavering, wavery,
   wavy, wayward, whimsical, wishy-washy, without warning