fitfulness vs unregularity

fitfulness and unregularity both are nouns.

Word	Noun	Adjective	Verb	Adverb
fitfulness	Yes	No	No	No
unregularity	Yes	No	No	No

As nouns, unregularity is a hypernym of fitfulness; that is, unregularity is a word with a broader meaning than fitfulness:

fitfulness: the quality of being spasmodic and irregular
unregularity: not characterized by a fixed principle or rate; at irregular intervals

Other hypernyms of fitfulness include irregularity.

fitfulness (noun)	unregularity (noun)
the quality of being spasmodic and irregular	not characterized by a fixed principle or rate; at irregular intervals