If you suspect modem malfunction, check for heat. Replace the modem if you can. If it is an internal modem, change slots and try running with minimum other internal cards. (Remove SCSI cards, Soundblaster cards and so on.) This is to verify that the root cause is not a plain old Windows hardware conflict.
If all else fails, it may be a marginal card in the CO DSLAM that is causing your line problems. Suggest (although this might be ignored) that the DSLAM card be re-seated, or that you be swapped to another card.
At least one user has reported that not enabling DMA on his hard drive was the cause of his sync problems.
Also read About DSL for lots more information