A clock synchronization backup mechanism is disclosed for maintaining clock synchronization during periods of degraded synchronization. The clock synchronization backup mechanism includes a jitter buffer having a fill value at a given sample time which is compared with a threshold. When the jitter buffer fill value exceeds the threshold, a non-normal condition is registered and the local clock frequency is set to a combination of a long-term frequency setting plus a threshold sensitive frequency adjustment. The clock synchronization backup mechanism is particularly useful for overcoming residual errors accumulated due to temperature change, oscillator degradation, and a variety of other system perturbations problematical for clock synchronization mechanisms known in the art.