A pressurized window bomb arrangement for AP-HTPB combustion is studied using high speed images to analyze for unsteady combustion, resulting in combustion instability. The base combustion is unsteady and non-stationary. The base filtered heat release rate pattern displays intermittency, seldom observed in prior literature. The intermittency observed is of type 1, and subsequent analysis leads to questions on conventional amplitude spectra as a measure of instability. A novel indicator, the intermittency factor is defined to understand the role of intermittency. It is seen that intermittency is marked by fluctuating intermittency factor values, implying a large deviation in phase values.