Intermittent sync on a modem can be caused by many different factors. If troubleshooting has been exhausted on both the CO and CPE side, you may want to check for line conditioning. Open or shorts on the line can cause signal drops. Defective DSL filters and modems can also do the same. Customers may have also connected unfiltered devices on the line that might be causing noise. 6 filtered analog devices would be the maximum for a single line. If a customer requires more than 6 devices on the line, it would be recommended to have a dry pair installed or a different splitter at the NID.