periodic

continual, intermittent, recurrent, recurring

cyclic (or cyclical)

around-the-clock, hourly, round-the-clock

chronic, confirmed, inveterate

expected, usual

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

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