1200Mbps plan but only getting about 20-60Mbps to the modem the majority of the day.

As the title suggests, I have the 1200Mbps plan, and it's generally been pretty good the last few years until about a few weeks ago when I noticed very slow downloads of a driver that I was getting from Nvidia.  I went to the app and ran some tests, soft booted the gateway about 10 times, nothing.  Getting between 1-5% of plan speed.  I then hard rebooted the gateway and was getting about 1400Mbps briefly for a few hours until it dropped again to below 20mbps.  I don't generally use WiFi.  I am always on Ethernet by the way although my wife does use the Wifi and I have a bunch of gadgets connected.  Occasionally, late at night I can get 200-400Mbps but it's for very brief periods of time.  Seems like all my neighbors are running some sort of peer-to-peer network or something.

Either my neighbor is stealing my bandwidth (I just shut off the hotspot WiFi, no change) or I have a nicked cable.   I need this fixed.  Wife works from home and I sometimes need to connect from home for work.  In any case, my bill keeps going up and now paying almost $300.00/mo. the service should be perfect.  This is going on over a month now.  The fact I can't get support over the phone, and have to download a phone app to check my services (Xfinity desktop webpage is no longer in service) is frustrating.  I hate using a mobile phone for these things.  Have an appointment for Saturday hopefully they show up.  If anyone has a utility where I can check whether my neighbor is tapping into my service, that would be great.  Should I install a MoCa filter?