intermittent

alternate, alternating, cyclic (or cyclical), rhythmic (or rhythmical), seasonal, serial

aperiodic, erratic, fitful, irregular, occasional, spasmodic, sporadic, spotty, unsteady

convulsive, sudden, violent

broken, disconnected, fragmentary, interrupted

aimless, arbitrary, desultory, haphazard, hit-and-miss, hit-or-miss, odd, random, scattered, slapdash, stray

capricious, changeful, changing, flickery, fluctuating, fluid, inconstant, mercurial, mutable, temperamental, uncertain, unpredictable, unsettled, unstable, varying, wavering

changeable, fickle, variable, volatile