Really variable internet connection?

[134]

My internet connection has been really variable lately. One second it will be fine and the next it will just nearly stop functioning. I don't see why either, granted we have a lot of stuff connected but our router is top of the line. We get Charter Hi-Speed 30Mbps, and our new router is the Netgear N750 Dual Band Gigabit Router. Our modem is a couple years old old, DOCSIS 2.0, but I don't know why this connectivity happens. One second I will have fast speeds the next I won't have any connection or abysmally slow connection. Our cordless phones run on 900 Mhz frequency. Interference isn't much of a problem where we are. What is the issue?
I mean our phones use 1900 Mhz