This study draws on conceptual and methodological insights afforded within a dynamic systems perspective to explore shifting interrelationships between cognitive capacity and motivational resources in instructed adult second language (L2) learners of Spanish at increasing proficiency. Relationships that emerged showed both stability and fluctuation over a semester of instruction and varied by learners' stage of development. Findings support previous calls to improve upon mainstream approaches to conceptualizing and investigating learner individual differences (IDs) by adopting a holistic, non modular view of the L2 developmental system in general and learner IDs in particular (e.g., Dörnyei, 2009b, 2010a]; Dörnyei & Ryan, 2015; Larsen-Freeman, 1997, 2001, 2015). To further align with DST principles (see Hiver & Al-Hoorie, 2016), future studies should endeavor to carry out longitudinal case studies with multiple data points in order to reveal intra-individual complexity in the group-level patterns seen here.