s

by fits and starts (irregularly; often stopping and starting again • He did his work by fits and starts); at odd moments; by fits and start; by snaps; fitfully (Aly19); in dribs and drabs (Anglophile); in snatches (Anglophile); intermittently (rucarut); fits and starts (Though placed on a low-cholesterol diet, he kept to it only in fits and starts. joyand); by fits; by catches; by snatches; by starts

at bits and starts (выражение вытеснено оборотом by bits and starts Bobrovska)

in fits and starts (Marina_Onishchenko); by bits and starts (Bobrovska)

in patches (Andrey Truhachev); once in a while (Andrey Truhachev); every once in a while (Andrey Truhachev); now and again (Andrey Truhachev); now and then (Andrey Truhachev)