Most likely the channel your modem is tuned into is saturated. Factory resetting the modem will allow it to pick up a new channel, and maybe that channel is less busy.
The factory reset button is on the back of the modem. You'll need a pen or paper clip to get to it. With power on, hold button in until the modem light pattern shows that it is rebooting. Probably best to do it in the evening when the problem is bad. It can take quite a while to get back online, maybe even half an hour.--