The Collaborative International Dictionary of English v.0.48:

Fit \Fit\, n. [AS. fit strife, fight; of uncertain origin.
   [root] 77.]
   1. A stroke or blow. [Obs. or R.]
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            Curse on that cross, quoth then the Sarazin,
            That keeps thy body from the bitter fit. --Spenser.
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   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.
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            And when the fit was on him, I did mark
            How he did shake.                     --Shak.
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   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.
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            All fits of pleasure we balanced by an equal degree
            of pain.                              --Swift.
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            The English, however, were on this subject prone to
            fits of jealously.                    --Macaulay.
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   4. A passing humor; a caprice; a sudden and unusual effort,
      activity, or motion, followed by relaxation or inaction;
      an impulsive and irregular action.
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            The fits of the season.               --Shak.
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   5. A darting point; a sudden emission. [R.]
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            A tongue of light, a fit of flame.    --Coleridge.
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   By fits, By fits and starts, by intervals of action and
      repose; impulsively and irregularly; intermittently.
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WordNet (r) 3.0 (2006):

by fits and starts
    adv 1: intermittently; "he worked on his book by fits and
           starts"