If in the Session Setup window in Pro Tools the Sync mode is set to "Optical" (meaning that Pro Tools is deriving a clock from the ADAT, via the optical connection), this can happen if the ADAT is not providing a clock.Make sure the ADAT is set to "Internal" sync mode, and that the Pitch Shift function of the ADAT is disabled, since if it is engaged it will provide a non-standard sample rate clock via the optical connector.Another solution is to have Pro Tools set to Internal Sync mode, and setting the ADAT to "External Digital" sync mode, meaning that it will RECEIVE a clock via the optical cables, and Pro Tools ill be the clock master.