This paper discusses measurements of the dynamic performance of electric power Phasor Measurement Units, PMUs, and their relation to the requirements of the IEEE Synchrophasor Standard C37.118-2005. In particular, it proposes a new method for monitoring the dynamic power signals that change in amplitude and frequency during the testing of PMUs. The new method estimates the reference values for dynamic power signals so they can be compared with the PMU values to determine the PMU's errors. This requires estimating the "true" timestamped values of dynamic power signals with low uncertainty. The paper also discusses the issue of aliasing of sampled dynamic signals, and the relationship between the requirement in the Standard to reject an out of band or interharmonic signal and dynamic signal performance.