Roget's Thesaurus: Entry 139 (Irregularity of recurrence)

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#139. Irregularity of recurrence. -- N. irregularity, uncertainty, unpunctuality; fitfulness &c. adj.; capriciousness, ecrhythmus|.

Adj. irregular, uncertain, unpunctual, capricious, desultory, fitful, flickering; rambling, rhapsodical; spasmodic; immethodical[obs], unmethodical, variable.

Adv. irregularly &c. adj.; by fits and starts &c. (discontinuously) 70.