on certain occasions (adjective) periodic: at fixed intervals
Detalii: periodic means happening or recurring at regular intervals; sporadic means occurring singly or occasionally or in scattered instances
Sinonime: arguable,
controversial, debatable, disputable,
doubtful,
dubious, dubitable, fishy, hard to believe, iffy,
indefinite, open to doubt, open to question,
suspect,
suspicious,
uncertain, unconfirmed, uncorroborated, unsupported
Antonime: constant,
continual,
irregular,
lasting,
permanent, sporadic,
variable