If something happens repeatedly, but irregularly, at uneven intervals, can it be described as “periodic”? I know the strict sense is regular, especially in math or science. But uneven intervals are still intervals. And if someone colloquially refers to their “periodic” house parties, which come around at intervals of anywhere from 6 weeks to 18 months, I’m not apt to complain.
You might be looking for sporadic:
occurring at irregular intervals or only in a few places; scattered or isolated.
"sporadic fighting broke out"
synonyms: occasional, infrequent, irregular, periodic, scattered, patchy, isolated, odd; intermittent, spasmodic, fitful, desultory, erratic, unpredictable; nonconsecutive
"partly cloudy with sporadic showers"
I like Workman's answer because it clearly implies that "periodic" should be limited to fairly regular occurrences. (I concede that many people do use the word "periodic" to mean "intermittent." I just see no reason to drain every word of a precise meaning because some people are lazy or uninformed.)
I prefer "intermittent" or "unpredictable" over "sporadic" as a general term because "sporadic" seems to imply infrequent or limited occurrences whereas "intermittent" and "unpredictable" make no implication about frequency or severity.