A recent theory for the time dependence of inhomogeneous line shapes is extended to account for fluctuations of the solvent response times tau in both space (heterogeneity) and time (rate exchange). Different simulation techniques are outlined for solving the Ornstein-Uhlenbeck type spectral diffusion in the situation expected for supercooled liquids. For intrinsically exponential solvent dynamics, slow rate exchange (or static heterogeneity) has to be assumed in order to reproduce measurements of the Stokes-shift correlation function C(t) and inhomogeneous linewidth sigma (t) simultaneously.