Internet Randomly Drops - Performance Drops

I've had a very frustration experience with Cox internet on this issue. For the past two weeks, I've had random drops in my connection and most of the time it slows down to the point to where my router stops working and everything goes down. I've tried the following:

1. Multiple reboots and resets with and without Cox support on the phone. 

2. Connecting directly to modem from PC - I can see the issue when I bypass the router. Connected 2nd PC to the modem directly to modem as well - same issue. 

3. Replaced the modem - problem went away for about 5 days - then came back

4. Had cox support out - they tested the line from the street to the router - changed some fittings and cables - all tested fine, signal was good going to router.

5. 30min after the cox support guy left it all went back off line. 

6. This morning it was working again so I connected the router and I've been able to stay connected for two hours - during that two hours I happened to be next to the cable modem and watched it suddenly go off line and reboot itself. It came back up and everything was back on-line. 

More detail: usually I notice when running speed test that the speed comes way down when the issue comes back - it goes from ~300 down using speed test to about ~80 down, and then ~15 down. Also, the ping goes from sub 10ms to about 50ms and eventually, the ping times out when I run speed test so that I can't even start the test. So it seems like the connection usually never fully goes out, it just gets really bad to where nothing works. 

Although everything is up and running as I type this, I'm sure its bound to all come crashing down again soon - any suggestions or ideas on what it might be?

Thank you