Intermittent slow Internet speed issues for months now, Gig service.

I pay for 1200 Mbps Internet.  Since signing up, I've had intermittent speed issues where on average it drops to 500ish Mbps, but today it hit 50.   

I'm not constantly running speed test either to try catch it, it's noticable.  Wired devices show Pixelation with YouTube, Call of Duty will get jittery with really bad lag.  I've run Internet speed test going to Comcast and see the same issues.  The Xfinity app shows the same speed as well.

I've opened multiple tickets with support.  They replaced the line to the poll and installed a new drop.  Everytime they come out, they find some small things and say it's fixed, and it runs a bit better, but the problems come back.  The tech will usually say that he ran a test and and everything looks good, but when he checks onsite, he sees some signal problems but says it wouldn't be enough to cause any issues.

I've replaced the modem, the internal cabling and they had me unplug a router that I was using that support said would cause issues, but still no change.  

What can I do at this point to get reliable service?