Problem solving in urgent situations challenges decision makers to make sense of their environment and take action quickly.  This paper draws on novel data from an in-depth study of clinical problem solving in operating room crises to develop a grounded theoretical model of diagnostic decision making under time pressure.  We develop a system dynamics model that represents the interactions among acting, interpreting cues, and cultivating new diagnoses.  The model replicates four dynamic patterns of diagnostic sensemaking observed in the source data: stalling, fixating, vagabonding, and adapting.  We show that self-fulfilling interpretation gives rise to fixating, but paradoxically is also crucial to adaptive sensemaking.  By exploring the critical interactions among acting, interpreting cues, and cultivating new diagnoses, we find that generating new diagnoses quickly may be a liability and that, in diagnostic problem solving, acting quickly can mitigate the effects of slow interpretation.  Driven by powerful reinforcing dynamics, the effectiveness of time-pressured decision making depends on the interplay among these processes.  We discuss model-based strategies for mitigating the failure modes observed in the source data.