Identifying the location of performance bottlenecks is a non-trivial challenge when scaling n-tier applications in computing clouds. Specifically, we observed that an n-tier application may experience significant performance loss when bottlenecks alternate rapidly between component servers. Such rapidly alternating bottlenecks arise naturally and often from resource dependencies in an n-tier system and bursty workloads. These rapidly alternating bottlenecks are difficult to detect because the saturation in each participating server may have a very short lifespan (e.g., milliseconds) compared to current system monitoring tools and practices with sampling at intervals of seconds or minutes. Using passive network tracing at fine-granularity (e.g., aggregate at every 50ms), we are able to correlate throughput (i.e., request service rate) and load (i.e., number of concurrent requests) in each server of an n-tier system. Our experimental results show conclusive evidence of rapidly alternating bottlenecks caused by system software (JVM garbage collection) and middleware (VM collocation).