My router runs 2 networks. A 5 GHz network for newer devices, and a 2.4 GHz network for older devices. When I am downloading a large file and/or have a lot going on, if I am connected to the 5 GHz network, both networks disappear for about 10 seconds, then re-appear. I have a Linksys E3200 router.
This suddenly started happening about a month after we reset our router.