Fitful, spasmodic, convulsive are comparable when they mean lacking steadiness or regularity in course, movement, or succession (as of acts or efforts).
Fitful stresses variability and intermittency; it implies an irregular succession characterized by fits and starts.
Spasmodic implies fitfulness, but it further suggests marked alternations (as of violent activity and inactivity or of great effort and of negligible effort or of zeal or enthusiasm and lack of interest); it therefore implies, even more than fitful, an opposition to what is sustained at a high pitch.
Convulsive differs from the preceding terms in not implying intermittency and in stressing unsteadiness, strain or overstrain, and the lack of such regular rhythm as is the sign of control and especially of muscular, mental, or spiritual control.