What type of device is sourcing the OC3? Do you trust it, know its timing traceability, does it provide SSM, are there optical level errors, is the link stable, etc?

You should be able to check it with an optical test set and look at the PM's.

If you have an alternate optical interface, you can run a test by using that as the timing reference. Just make sure to not create a timing loop in doing this. This should eliminate the internal clock as being bad.