I would look at the logs on the MODEM and check to see if there are any issues seen. I would also check the phone line going from out side to the MODEM. Make sure they are in GOOD working condition. Ensure the phone lines are correctly filtered. Have the ISP check there logs for disconnections during this time frame. Have them check the lines from outside going to there point of connection. Ask the ISP if there is any FW updates that they are responsible for and if any is available.