This paper discusses the formal representation of morphosyntactic intra-speaker variability in a modular grammar. It presents novel data from Indonesian functional prepositions that exhibit unusual variability in form. It is shown that oleh ‘by’ and dengan ‘with’ may either (i) be phonologically unrealized (Preposition-drop), a process sensitive to syntactic structure, or (ii) may be realized as sama ‘by/with,’ a variant speakers use in an informal style. We establish that it is necessary to invoke distinct processes to formally introduce these outputs: Indonesian preposition-drop is argued to reflect a morphophonological variable deletion rule, while style-shifting is modeled as competition in use of lexical inventories. The variable realization of Indonesian prepositions indicates that it is possible to identify distinct variable processes (competition and variable rules), here operating over a single morphosyntactic item. The appeal to two variable processes follows in part from the nature of the factors (internal or external to the grammar) that condition variability, but we argue that it also follows if implementation additionally obeys constraints imposed by the grammatical architecture. Finally, we find that, in order to maintain modularity, variable processes may have distinct loci in the post-syntactic derivation.