We're running multiple leased-line DS-3s in our backbone. Recently I've spent several weeks troubleshooting one DS-3 going down for an hour or so every few weeks. CRCs on both ends. The long haul carrier saw frequency errors of -88ppm (20ppm is acceptable). There are multiple carriers in this link, as well as our other DS-3s.
Card swaps did not alleviate the errors (no h/w problem).
Carriers are all running Strata 3 clocks. Carrier and I agreed to change from 'clock source line' to 'clock source internal' on one router. Problem solved.
Carrier mentioned to me they see this more and more and attribute it to the many new carriers and errors in Strata 3 synchronization among them.
I'm considering changing all backbone DS-3s to internal but am interested if anyone else is seeing this problem before I re-provision these links.