This study examined how students' interest, self-efficacy, and perceived difficulty change during a task, how those changes relate to each other, and how they predict performance. Sixth-graders (N = 1024) rated their interest, self-efficacy, and perceived difficulty repeatedly during a dynamic problem-solving task. Results from the estimated non-linear and piecewise latent growth curve models showed interest and self-efficacy to decrease, and perceived difficulty first to increase, and then to decrease, over time. The levels of and changes in interest and self-efficacy correlated positively with each other, but negatively with perceived difficulty. Task performance was positively predicted by initial interest and less negative change in self-efficacy, and negatively by initial perceived difficulty and steeper increase in it. The results suggest perceived difficulty to have a distinctive role in the dynamics of task-specific motivation, and on-task changes to be relatively independent of more general motivation and competence.