Ethernet stops working - looks like there is an IP conflict

I have two PCs plugged in via ethernet in my house. Connection is running from my Xfinity Gateway to a switch with 4 ethernet ports in it. 2 ports feed into wi-fi extenders and 2 of them feed into PC in my office and PC in my living room. 

If I have PC 1 on and turn on PC 2, both will disconnect and be unable to access the internet. When I run the troubleshooter on Windows it says I have an issue with my IP address, looking like there is an IP conflict. On both PCs I've tried running command prompt and release/renew my IP. 

Wifi still works, my wifi extenders still work. Just the PCs plugged into ethernet stop. 

Unplugging the gateway and waiting a couple minutes always fixes it so far. Both PCs will work until the next time I reboot them, then I have to repeat the restarting of the Gateway. Not a huge deal, just wondering if there may be a known fix.

just some more details, PC 1 has a wifi card in it, and I sometimes notice the ethernet port receiving internet data and switches to wifi. Before I got PC 2 I thought it was just an issue on my PC 1.