IP Conflict but no ones there…

I have a 100 node isolated network on a dumb hub. Two addresses went “Bad”. This time, overnight the network traffic went from 17% to 59% and collisions doubled. Switching the IP address of one node fixed the problem, but a ping says no PC is onthat address. Assigning another PC to that address repeats the problem. This is a mixed network of PC’s and PLC’s, where I can not shut down to test except on scheduled outages. How do I find whatever is causing these conflicts?