My Internet Speed Keeps Dropping

My Cox Internet speed keeps dropping! We've had this problem for months now and have contacted Cox phone support numerous times. It all started one day when the internet speed suddenly dropped. We contacted Cox and they restarted it on their end. Then a few days later we had the same problem. So we contacted them again and scheduled them to come to our house and try to fix it. When they came, all they did was change the cable that connected the modem to the interwebs. Everything seemed fine and dandy until the internet started to drop again. So we called again, but this time they told us to update the firmware on the router (Note we were able to find out that the modem had nothing to do with the problem). So I did. And what did that do? Nothing. The internet was dropping and going up and then dropping again. So we got fed up with it and just bought a new router. Well it all seemed fun and games for a few days until the same problem started occurring. With a new router. So now we sit here as I type this message to try to get some professional help. Please do whatever you can I just want to fix this problem. Sorry if I didn't give enough info as I am a simple child with the mental capabilities as, well, a child. PLEASE RESPOND ASAP.

~SumJuan