In the past, flame geometry was found to play an important role in determining when strong pressure fluctuations associated with a combustion instability occur in a gas turbine model combustor. The goals of this study were to use flame surface area as a more accurate way to quantify the flame shape and the heat release rate, in lieu of chemiluminescence; to accurately resolve both the spatial structure and the time history of the heat release fluctuations, and to show that kilohertz formaldehyde planar laser-induced fluorescence provides a new way to achieve these goals. The dual-swirl burner, developed at DLR Stuttgart by Meier et al. was operated using dimethyl ether (DME) to study fluctuations in the flame surface area, flame brush, flame length, and heat release rate. To understand the instability, accurate measurements are needed of the correlation between heat release rate fluctuations and pressure fluctuations. Thus heat release rate must be recorded as a function of time and space. However conventional chemiluminescence offers only a line-of-sight measurement. High-speed CH₂O PLIF was applied to study the motion of flame surfaces in response to the pressure oscillations. Results show that the flame surface density fluctuated at the acoustic frequency (typically 320 Hz). The frequency of the combustion instability was found to increase as either the gas velocity or the flame speed was increased. These trends are consistent with previous n-τ theories that model the convective time delay. Strong pressure oscillations occur for conditions when the flame is nearly flat; the instability disappears when the flame becomes V-shaped. The explanation is that all points on a flatter flame have the same convective time delay but this is not true for the V-flame geometry.