HELP!!! R6300 Keeps loosing internet

I am going crazy! We had a D-Link DIR-655 & I was getting internet on every computer but my one main PC. I did everything I could think of and the only way I could get internet on that PC is by direct line to the modem (cable). I figured it was the router, so I got a new one - Netgear R6300. Set it up & could get intermittant internet - now on all, as the Genie seems to have taken over and when there is no connection to the main PC, there is none to any, despite the PC saying that there is internet, the router says there isn't (and there isn't). I then thought maybe it was the modem so I took that in and exchanged it (it's an ARRIS). Still, same problem. We'll get internet for about 5 minutes and then booted off - the genie then does a reconnect and I get this message:


I am now directly connected from the main PC to the modem in order to stay on the internet. I am going bat-$#!% crazy over this! I am not a tech & really do not know tech support stuff very well, so please use specifics in any advice give. I have a cable tech coming out tomorrow, but I'm afraid he's going to say there's nothing wrong, since the modem continues to have connectivity. HELP!!!!