The Phrase Finder was once again where I located a definition. Several sources described the expression to mean behaving in an impulsive or irregular manner or in irregular bursts.  The word “fit” can be used to describe a medical condition called a paroxymal attack.  There are sudden spasms or convulsions caused by multiple sclerosis, head trauma, epilepsey, malaria, and several other conditions.  Fit can also be used to describe an emotional reaction. William Warner wrote in Albion’s England in 1581, “His seruants fear his solemn fittes.” Fits and starts both can mean sporadic activity and the expression was first recorded by Robert Sanderson in Sermons in 1681, “if thou hast these things only by fits and starts.”