Fits and starts
Spasmodically; at irregular intervals.
One of the many meanings of 'fit' is a paroxysm, or sudden and temporary seizure. The meaning of 'fit' when 'fits and starts' was coined, in the 17th century, was an earlier but milder version of that. It was then used to mean 'a sudden state of activity (or inactivity) or state of mind', as in this early example, taken from William Warner's Albion's England, 1586:
"His seruants fear his solemn fittes."
The phrase 'fits and starts' is somewhat tautological, as both allude to sporadic activity. It is first recorded in Robert Sanderson's Sermons, 1681:
"If thou hast these things only by fits and starts."