The present work deals with time-resolved investigation of the flow field during acoustic self-excitation by a lean premixed flame in a dump combustor. Simultaneous measurements of unsteady pressure, velocity fields using time-resolved particle image velocimetry (TR-PIV) and CH* chemiluminescence are performed. As the equivalence ratio is varied, conditions of maximum pressure amplitude correspond to the prevalence of intermittent bursts in all the measured quantities. The intermittency is quantified by means of the permutation entropy of the pressure signals. The most significant frequencies are identified with the fundamental natural duct acoustic mode and the large-scale vortex roll-up (“wake mode”) frequency. The simultaneous occurrence of these multiple frequencies and shifts in their relative dominance of frequencies over time, shown by wavelet transform, result in the observed intermittent burst oscillations. Regimes of these ordered oscillations alternate with those of relative silence as well. The dynamic mode decomposition (DMD) of the TR-PIV data during windows of decay and growth of intermittent oscillations in the pressure signal quantifies the contribution of the above dominant modes in terms of their temporal growth rates. Besides the above two modes, the flapping mode and the shear layer mode of the flow field are also indicated. Consecutive PIV realisations and chemiluminescence images in a time-resolved sequence during excitation of high pressure amplitude show the large-scale vortex roll-up to substantially convolute the flame, leading to high heat release fluctuations. A further sequence of consecutive flow/flame fields is also examined as to how oscillations emerge out of a relatively silent time interval. The Kelvin–Helmholtz structures of the shear layer mode prevalent during the silent regime are seen to undergo a collective interaction to form a large-scale structure that excites the vortex and acoustic modes in turn. © 2016 The Combustion Institute