by fits and starts

In short, inconsistent, and irregular intervals, as of motion or progress. The car was almost completely broken down, but, by fits and starts, we were able to get it to a mechanic. A: "How's the essay coming along?" B: "Oh, by fits and starts." A: "Why is your suitcase still half empty?" B: "Don't worry, I'll be ready in time—I just pack by fits and starts."

fits and starts, by

Also, in fits and starts. With irregular intervals of action and inaction, spasmodically, as in The campaign is proceeding by fits and starts. This expression began in the late 1500s as by fits, the noun fit meaning a "paroxysm" or "seizure"; starts was added about a century later.

by

/in fits and starts

With irregular intervals of action and inaction; intermittently.

by

/in virtue of

On the grounds or basis of; by reason of: well-off by virtue of a large inheritance.

American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

fits and starts, by

In bursts of activity, spasmodically. The fits portion of this expression dates from the sixteenth century, and the pairing with starts came soon afterward, in the early seventeenth century. “Thou hast these things only by fits and starts,” wrote Robert Sanderson in one of his Sermons (1620). John Ray’s proverb collection of 1670 put it slightly differently: “By fits and girds, as an ague takes a goose.”