I found the problem. A security camera DVR on the network had failed and when it powered up it went to its default IP address which is 192.168.10.1 - the same as my router. Depending on the timing of the client's arp query some would register the 192.168.10.1 as the router and some would register it as the DVR. The clients that registered the MAC address of the DVR in their arp table would not connect to the internet.
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