My guess is that the ADAT cable is not always going fully home, and from time to time you are getting a marginal ADAT signal. This was probably always the case, just it did not cause problems.

It does not look to me as though your Wordclock has a problem, but note that the terminator needs to be on the last INPUT in the chain (if chained, otherwise on the INPUT). See the section "Wordclock Termination" in the UM for discussion on this point.