I had something similar happen and when I read the error log in the router I kept seeing a message over and over about ports trying to be opened by some IP address. That IP address ended up being a wireless camera the customer bought and put on the network. I disabled the UPNP setting in the camera, the messages went away and the router stabilized. I'm not a super strong IT guy, so I'm not sure if that was the right way to approach it, but it worked.