Problem Line down with RT-AC68U

I get an Asus RT-AC68U with 380.67 firmware, it happens that at least once a day if not 2, I no longer surf the Internet.
I thought it was the modem, but after testing I saw that switching off/on the router the line went back and this never happened to me before. When happened its impossible to to main page of router (192.168.0.1)
How can I fix it .. i can downgrade firm ?

Thank
Hispa