external clock is invalid. reverting to internal..

[lowlight]

I have my 002 rack synced to incoming s/pdif time code via a motu digital time piece. it works well, but I get this error message way too often (several times per hour of recording):

The external clock choice is invalid. reverting to internal.

If I just keep resetting the clock setting (either in the hardware setup dialog or in the session setup dialog) back to s/pdif, it eventually accepts that and runs again for a while, but damn I'd really like to be able to work without these spates of errors. My sync setup works perfectly - I get locked tracks on tape and disk for as long as I want - anyone have any experience with this?

please chime in -