every once in a while

annual intermittent monthly occasional recurrent recurring regular repeated routine seasonal sporadic weekly yearly

periodical

alternate at various times centennial cyclic cyclical daily epochal every so often fluctuating hourly infrequent isochronal isochronous on certain occasions on-again-off-again orbital perennial rhythmic serial spasmodic

constant infrequent unusual

continual irregular lasting permanent sporadic variable

every so often now and again now and then

ever so often from time to time once and again once in a while sometimes

ever so often every now and then infrequently not often now and again occasionally once and again once in a while periodically sometimes sporadically

every now and then every so often infrequently intermittently now and again occasionally periodically sporadically

at intervals at times from time to time hardly irregularly not often off and on on occasion once and again once in a blue moon once in a while once or twice sometimes

alternately at intervals at times every so often fluctuating from time to time irregularly now and then occasionally on occasion once in a while sometimes sporadically vacillating variably

at intervals at times discontinuously every now and then every so often from time to time hardly infrequently intermittently irregularly now and again now and then occasionally off and on on occasion once in a blue moon once in a while periodically sometimes sporadically