I had the same problem with and older Netgear router. It turned out to be a bug in the scheduling settings. It had the ability to disable network access between specific times on a daily schedule. The feature was enabled by default, but no times were set to disconnect. You'd think that would mean the connection would never go down, but a firmware update changed that. Every morning at ~3:00am the network would go down for 1 minute. Disabling the feature entirely resolved the problem.