If you have them connected to a power strip/surge protector, that could be causing it. If I were you though, I would call comcast and ask them to diagnose your modem to see if they notice any degradation.
You could also go into setup https://192.168.1.1 for your router and set up a MAC filter and set the frequency to automatic. If it's stuck on one band, there could be interference that's causing you to lose connection.
usually disconnecting and low number of bars is because you need to change channels. I would change the channel several levels higher or lower. dont do just one channel level higher do several.