aperiodic
/ˌeɪpɪriˈɑdɪk/
adjective
Not periodic; irregular.
aperiodic fluctuations
irregular,uneven,varying,variable,erratic,jerky,unreliable,volatile,spasmodic,sporadic,changeable,changing,fluctuating,wavering,vacillating,inconstant,intermittent,fitful,desultory,occasional,unsystematic