Internet constantly fluctuates Mbps

I have had Xfinity Home Internet for several years and have never had any issues with it. I've always been able to stream content and play online with no issues at all. However, up until recently, I've started having issues with the service. About 2-2 1/2 weeks ago I started having issues streaming content and playing online. My Mbps constantly drops, and I'm not talking about it dropping low and staying low, it constantly fluctuates. It will go from 150 Mbps, down to 6 Mbps, up to 20 Mbps, back down to 11 Mbps, etc...this happens non-stop throughout the day. I anticipated that it was an issue with my Gateway as it was an older model one so I ran to my local Xfinity store and swapped it out with a newer one. I still had the same issue. I even swapped out the coaxial that runs from the gateway to the wall and checked all the connections and everything looked fine. I eventually ran back to the store and scheduled for a tech to come out and see if there's an issue with the outside lines/connections (I live in an apartment). I was told that everything looked fine. He reset the gateway and verified that everything was working as it should. It almost seems like I'm receiving the signal that I should be getting but for whatever reason it's not putting out the signal I should be getting. Long story short, they sent me another gateway, this is my third one. I plugged it in and had it activated, the problem continues. I am at my wit's end. I don't know what else to do....Xfinity tells me everything is fine and that there are no problems but my experience suggests otherwise!?! Has anyone else had this issue and gotten it resolved? If so, how? or can anyone think of what else the issue may be? Please help.