I am trying to set up ADSL and WiFi in a country home. I have bought a DLink DSL-2640 modem-router for this. The device is connected to the ADSL, everything works fine. The device has a WiFi AP capability - I have set up the WiFi and I am accessing the network from 2 different iPads, 2 PCs and an Android phone. After a few hours of work I begin to notice that one of the iPads is taking to long to load pages - it even can't load the router's web interface. This goes on for 5-10 minutes or so, then it passes away. Same stuff happens to Android, but rarer. PC has this problem occasionally also, but rarest of all.
I am assuming that this is a problem of the device - ADSL and WiFi might be too much for it. I got a dedicated WiFi router (TP-Link WR740N), connected it to the DLink and turned off WiFi on the D-Link.
Same stuff happens. After a while the Internet stops responding - I can't load the router's web interface. Goes on for 2-10 minutes and passes. WiFi is functional, I can disconnect and connect a device, device obtains an address through DHCP.
I updated both devices to the latest official firmware. I tried switching WiFi channels, switched the router to g-only mode. EM interference is very unlikely since it's a country home and there are no wifi networks around.
I am at a loss and don't know how to debug this further. Any advice is welcome.