Wifi issue

 I have an Arris SB8200. I previously changed my speed to Gigablast, but ran in to a lot of issues, and switched back down to 300 mbps. Ever since I switched back down, I have been having constant issue with my Wifi. It runs really slow at times (buffering, disconnections etc). I have had a couple techs come out and hard reboot my modem and router, and have been told the issue should be resolved. I am kind of at a loss to what the issue might be. Every time I call, i'm told that there are no signal issues. I have swapped out my modem and router a few times, but still no fix. Could it possibly be the modem that could be causing the issue?