losing connection to network when downloading

Whenever I start the download of a video game, through the Steam client for example, the download will start but as soon as it does everything else connected to my router disconnects. The download will continue just fine for however long it takes to finish, but while the download is in progress everything else that is connected to the router, ethernet or wifi gets kicked off. My ISP was less than helpful and says it must be a router issue. I  have a "fiber" connection and do not have a modem, my ethernet cable runs straight from my wall jack to the router. I've looked through my router settings but seem to have no luck finding anything that looks like it would help. Any advice is appreciated.