Cable Modem Keeps Losing Connection

Just got high speed internet  300 and the signal keeps dropping out.  It works fine for awhile then - nothing.

I have spent literally hours with Cox Reps testing the line-to no avail.

I have also spoken to modem manufacturer and they did tests.

I even exchanged my brand new modem for another just in case it was that

It's not the router because tthe computer stays hooked up to the wi-fi.  The modem is hooked up to the coax

coming out of tthe wall (abut 3feet)  There is no splitter.  there is a a wifi router connected to the modem.

Cox said they would send a tech to check cable from their box to my house

I never heard back from them.

If I unplug the modem and undo cable then reattach it comes back on in about 1-2 mins

Modem:Arris Surfboard SB6183 DOCSIS 3.0 Cable Modem 686 Mbps 16x4

Router:NetGear AC1200 Dual Band WiFi Router300+900 Mbps

I notice alot of people have had this problem but it desn't look like they get any answers.