Here are some more steps you can take to check and isolate any inside problems:
Try a different line cord from the modem to the phone jack.
Unplug all other phone devices on the line. If performance changes, plug them back in one by one to isolate the problem. To verify it is a particular device and not the jack or the wire to the jack, plug the problem device into another phone jack, and check its line cord.
In some cases, EMI (electro-magnetic interference) may make its way into your phone wiring. Sometimes it's as simple as re-routing the line cord from your modem to the jack.