This study investigated fMRI Blood Oxygen Level-Dependent signal variability within individuals (within-individual neural variability; WINV) during a letter n-back task, and examined the relationship between WINV and cognitive abilities in healthy adults (N = 48). WINV in frontoparietal brain regions was modulated during vigilance and working memory (WM) trials of the n-back task, and was related to neuropsychological measures of vigilance and WM. WINV across the n-back task influenced n-back task performance; in this study, the inferior frontal junction exhibited a behavioral double dissociation between flexibility and stability at the region of interest level. A reading and language network was also queried to determine the influence of vigilance and WM on reading and language skills. As hypothesized, ROI and cluster-based variability in the n-back task was related to performance on assessments of WM, vigilance, reading, and language. Understanding WINV in these domains will inform research about WINV in clinical populations.