Internet drops after 15 minutes, light on gateway still shows connected

Where to begin?  Have had Xfinity for about a year and it's worked fine - Performance Pro plan, I believe.  Normally get around 150-250mbps.  2 days ago, the connection dropped and I had to unplug the gateway, wait a few and then plug it back in.  It then worked for some time but then the internet went down again.  No wifi and nothing via ethernet cable.  The status light on the gateway remained white, meaning it was "working."  When I tried to reset via the app, I couldn't sign in for some time and then, when I finally did, it couldn't reach the gateway for an app reset.  

Techs came on yesterday, secured some wiring outside that they said was a bit loose, added some kind of connecter (not sure what kind) and then reset the gateway.  It worked for about an hour and then went down again.

Another tech came out yesterday afternoon.  The swapped out the gateway I had for a newer Xfi 3rd generation gateway.  He also checked some of the wiring and said it should now work.  The tech hung around for about 20 minutes to make sure and it worked.  Of course, when he left, the internet went down about 30 minutes later.  I did a reset again (app couldn't connect so had to unplug/plug in).  This time, it stayed up for a bit longer - several hours - but connection speeds were slow.  Normally, there is enough bandwidth to have a tv streaming in the living room, another one in my office and then all the devices to connect.  Every streaming device was lagging dramatically.  

The tech from yesterday afternoon came out again today.  He ran new wiring, drilled a new connection through the wall and checked everything out and said that was the only other thing it could be.  However, about 20 minutes after he left - it went down again.

I'm not sure what the issue could be at this point.  The light on the gateway consistently stays white, even when there is no connection.  My PC shows that it's connected (via ethernet cable) and even my phone shows the wifi connection - but nothing loads.  It does not come back up, either - with the exception of last night - it would slow down intermittently and then speed up a little bit.

Yesterday, I tried resetting the gateway to the factory settings and setting up the wifi again after the first techs came - but it was still going down.  The only thing I noticed is that, after the first techs came yesterday, when I went to reconnect to the wifi on my iPhon3, the network had a little note about advanced security on it.  I couldn't figure out how to get it off - I went to the Xfinity site and was unable to change it under the network/gateway settings.

After the tech came today, I restarted the gateway and went to the Gateway settings and I was able this time to turn off the advanced security settings.  So far, it's been about 20 minutes and it's still up, but I don't think that something as small and simple as that could be the root of this problem.

Any thoughts?  Is anyone able to help?  I messaged the tech that came out today to let them know but don't want to bug them.  This is just really frustrating to not have a consistent internet connection when I work from home and utilize the network in almost every aspect of my day-to-day life, especially given the current circumstances.

Any help would be appreciated!