DNS Resolver reverts to Comcast

I have my own modem/router so that I can use a custom DNS resolver on my network. I am finding that every day or couple of days the DNS resolver reverts to Comcast. Nothing changes about the configuration of the modem or the IP address, it just reverts. To get it back to my custom DNS resolver I reboot the modem and we are back in business. I'd rather not have to do that every day. Does anyone have any ideas about why this would happen?