'By fits and starts' definitions:

Definition of 'by fits and starts'

adverb

Intermittently; "he worked on his book by fits and starts"

From: GCIDE

Fit \Fit\, n. [AS. fit strife, fight; of uncertain origin. [root] 77.]
1. A stroke or blow. [Obs. or R.] [1913 Webster]
Curse on that cross, quoth then the Sarazin, That keeps thy body from the bitter fit. --Spenser. [1913 Webster]
2. A sudden and violent attack of a disorder; a stroke of disease, as of epilepsy or apoplexy, which produces convulsions or unconsciousness; a convulsion; a paroxysm; hence, a period of exacerbation of a disease; in general, an attack of disease; as, a fit of sickness. [1913 Webster]
And when the fit was on him, I did mark How he did shake. --Shak. [1913 Webster]
3. A mood of any kind which masters or possesses one for a time; a temporary, absorbing affection; a paroxysm; as, a fit of melancholy, of passion, or of laughter. [1913 Webster]
All fits of pleasure we balanced by an equal degree of pain. --Swift. [1913 Webster]
The English, however, were on this subject prone to fits of jealously. --Macaulay. [1913 Webster]
4. A passing humor; a caprice; a sudden and unusual effort, activity, or motion, followed by relaxation or inaction; an impulsive and irregular action. [1913 Webster]
The fits of the season. --Shak. [1913 Webster]
5. A darting point; a sudden emission. [R.] [1913 Webster]
A tongue of light, a fit of flame. --Coleridge. [1913 Webster]
By fits, By fits and starts, by intervals of action and repose; impulsively and irregularly; intermittently. [1913 Webster]