The power supplied to a high pressure, microwave sulfur plasma sustained by microwaves is interrupted on short timescales. The response of the molecular spectrum between 400 and 900 nm is measured during this interruption. Characteristic decay times of around 20 ms are found and are due chiefly to the large heat capacity of the reacting mixture. The results of a time dependent local thermal equilibrium model of the interruption, including radiation transport of the entire molecular spectrum and heat conduction, are also presented. The simulated signal decay agrees with experiment not only in magnitude but also as a function of wavelength.