The current paper studies the thermoacoustically unstable combustion, under elevated mean pressure, of a commercial swirl stabilized gas turbine burner fitted with optically accessible windows. The considered measurements include particle image velocimetry (PIV), OH∗ chemiluminescence imaging, high speed broadband flame imaging and dynamic pressure signals. We study cases A and B, wherein natural gas flames at mean pressures equal to 3 bar and 6 bar delivered thermal loads equal to 335 kW and 685 kW respectively. The flow field demonstrated a typical vortex breakdown induced inner recirculation zone and a sudden step expansion induced outer recirculation zone. In case A, high amplitude dynamic pressure bursts were observed amidst a quiescent acoustic background. The flame was conical, it anchored on the shear layers of the recirculation zone and it periodically expanded in the outer recirculation zone (ORZ). In case B, the flame was consistently thermoacoustically unstable with seldom requiescent events, while at the same time expansion to the ORZ was suppressed. By applying Dynamic Mode Decomposition on high speed images of case A, it was showed that this expansion introduced an additional time scale, further to the fundamental acoustically related timescale. The superposition of two timescales over a turbulent background established an intermittent regime of thermoacoustic instabilities, wherein the dynamics transitioned between quiescent and fully oscillatory. A physical mechanism is suggested to explain the differences between the flame shapes on adjusting mean pressure. The mechanism considers that the premixture is characterized by a Lewis number lower than unity, the laminar flame speed increases on decreasing mean pressure and the flow imposed on the flame strain rate oscillated over a period of thermoacoustic instability. This combination resulted in oscillatory heat release rate, in the region of the outer shear layers. The phenomenon was more pronounced in case A than in case B, because flow dilatation imposed-strain rates are higher for the former than for the latter flame. The paper argues that for a given fuel, elevated mean pressure introduces time scales that can significantly affect the dynamic regime the combustor operates in.