A swirl combustor with a dump plane is examined for its acoustic stability with partially premixed combustion. It is seen that the combustor exhibits intermittent oscillations at all operating inlet Reynolds number (Re), with the amplitudes switching between very high amplitudes (> 400 Pa) and high amplitude ( 200 Pa). Wavelet analysis of the non-stationary signals reveals the presence of a single band of oscillations, around 100 Hz. Further analysis reveals that at higher Re (> 25000), the high amplitude oscillations display a beat signature. The focus of the present work is on the high amplitude oscillations, contrasting the spatio-temporal behavior of the system for low and high Re. This is accomplished by employing a novel segmented Dynamic Mode Decomposition of time-resolved CH* chemiluminescence. The analysis reveals that at higher Re, the presence of beat signature is a result of spectrally closely spaced axial and helical modes. Such a feature is not seen at low Re, where only the axial mode is excited. An examination of coherence between heat release at different modes and pressure oscillation reveals flame asymmetry and spatial inhomogeneity of modes towards global heat release. © 2020, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.