The Collaborative International Dictionary of English v.0.48:

Sporadic \Spo*rad"ic\ (-r[a^]d"[i^]k), a. [Gr. ? scattered, fr.
   ?, ?, scattered, fr. ? to sow seed, to scatter like seed: cf.
   F. sporadique. See Spore.]
   Occurring singly, or apart from other things of the same
   kind, or in scattered instances; separate; single; as, a
   sporadic fireball; a sporadic case of disease; a sporadic
   example of a flower.
   [1913 Webster]

   Sporadic disease (Med.), a disease which occurs in single
      and scattered cases. See the Note under Endemic, a.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sporadic
    adj 1: recurring in scattered and irregular or unpredictable
           instances; "a city subjected to sporadic bombing raids"
           [ant: continual]

Moby Thesaurus II by Grady Ward, 1.0:

165 Moby Thesaurus words for "sporadic":
   aimless, amorphous, broadcast, broken, capricious, careening,
   casual, catching, catchy, chance, changeable, changing, choppy,
   communicable, contagious, desultory, deviating, deviative,
   deviatory, different, diffuse, disarticulated, disconnected,
   discontinuous, discrete, disjunct, disordered, disorderly,
   dispersed, dispread, disproportionate, disseminated, dissipated,
   distributed, divaricate, divergent, diversified, diversiform,
   eccentric, endemic, epidemial, epidemic, epiphytotic, epizootic,
   erose, erratic, few, fitful, flickering, fluctuating, formless,
   frivolous, gratuitous, guttering, halting, haphazard, herky-jerky,
   heteroclite, hit-or-miss, immethodical, impulsive, inchoate,
   incoherent, inconsistent, inconstant, indiscriminate, infectious,
   infective, inoculable, intermittent, intermitting, irregular,
   jagged, jerky, lurching, meaningless, mercurial, misshapen, motley,
   mutable, nonconformist, nonstandard, nonsymmetrical, nonsystematic,
   nonuniform, occasional, on-again-off-again, orderless, pandemic,
   patchy, periodical, pestiferous, pestilential, planless,
   pluralistic, promiscuous, ragged, rambling, random, rare, rough,
   scarce, scattered, scrappy, seldom, senseless, separate, shapeless,
   single, snatchy, sparse, spasmatic, spasmic, spasmodic,
   spasmodical, spastic, spotty, spread, spreading, staggering,
   straggling, straggly, strewn, strown, systemless, taking,
   unarranged, uncertain, unclassified, uncommon, undirected,
   unequable, unequal, uneven, unexpected, unfrequent, ungraded,
   unjoined, unmethodical, unmetrical, unordered, unorganized,
   unorthodox, unregular, unrhythmical, unsettled, unsorted, unstable,
   unsteady, unsymmetrical, unsystematic, ununiform, vague, variable,
   variegated, variform, various, varying, veering, wandering,
   wavering, widespread, wobbling, wobbly, zymotic