Problem of stability

Hello, I bought this router Asus RT-AC66U to change my old Netgear with Wifi g which crashed nearly everydays.
The router works well just after I start it, I have a good speed (15 mpbs for an ADSL connection) but 1-2 hour later, the speed become more and more slow and the ping increase. I reach 3 mpbs on a speedtest. Sometimes it works good and sometimes not, and I don't really understand why

I tried the following instruction from another post:

1. Update the firmware
2. After the firmware update, reset the router to factory defaults
3. Reconfigure the router from scratch
4. Remove all of your old wireless profiles from computers, phones, etc
5. Make sure 2.4 Ghz and 5 Ghz have unique SSIDs
6. For 2.4 Ghz, change the channel width to "20 Mhz"
7. Change the 2.4 Ghz channel with the help of inSSIDer
8. If you have cascaded your Asus router to another router, make sure it is not double natting on the same subnet. (Change the Asus IP address to something like 192.168.9.1) or use AP mode instead.

And I still have some problems :mad:

I post my configuration, if you see something wrong tell me