The evolution of the dynamic spectra of eight pulsars over 16 months is reported. Observations were made at Jodrell Bank at 408 MHz from 1984 September to 1986 January. The changing form of the spectra is interpreted as refractive modulation of diffractive interstellar scintillation. The correlation function versus time and frequency was computed for each observation and estimates were made of the apparent decorrelation widths in frequency and time, and the frequency drift rate of scintillation features. A model is developed for how these three parameters vary with epoch under changing angles of refraction. The model successfully describes the variations for four of the pulsars, for which the drift rate shows some sign reversals between epochs. Two pulsars kept a constant sign of drift rate, which we interpret as due to persistent refracting structures in their lines of sight, which are consistent with an intervening H ii region. Studies were made of the variability of the diffractive bandwidth, the visibility of the drifting features in the dynamic spectra, the refractive shifts and the speed estimated for the diffraction pattern. These quantities mostly agree with expectations from the model. The pulsar PSRB2016+28, which has an unusually low proper motion, shows clear modulation of the pattern speed by the Earth's annual motion. In the 11 epochs observed for PSRB2016+28, two showed unmistakable periodic modulation in the dynamic spectra. Several other pulsars exhibited occasional periodic patterns with low modulation. The phenomenon varies substantially between the pulsars, being more common for lower strengths of scattering. Key words: radiative transfer - pulsars: general - pulsars: individual: PSRB2016+28 - ISM: general.