WordNet (r) 3.0 (2006):

unrhythmical
    adj 1: not rhythmic; irregular in beat or accent [syn:
           unrhythmical, unrhythmic] [ant: rhythmic,
           rhythmical]

Moby Thesaurus II by Grady Ward, 1.0:

53 Moby Thesaurus words for "unrhythmical":
   broken, capricious, careening, catchy, choppy, desultory,
   deviative, disconnected, discontinuous, eccentric, erratic, fitful,
   flickering, fluctuating, guttering, halting, herky-jerky,
   heteroclite, immethodical, inconstant, intermittent, intermitting,
   irregular, jerky, lurching, nonuniform, patchy, rambling, rough,
   scrappy, snatchy, spasmatic, spasmic, spasmodic, spastic, sporadic,
   spotty, staggering, uncertain, unequal, uneven, unmethodical,
   unmetrical, unregular, unsettled, unsteady, unsystematic, variable,
   veering, wandering, wavering, wobbling, wobbly