ARP flooding on my LAN, how do i find infected computer?

I work at a small non-profit and about 20 staff and consultants come
and go throughout the day. One of their computers is infacted with
some sort of Malware that is attempting to spoof IP addresses on the
network which randomly kicks people off the network and the router
needs to be reset from time-to-time.

Any suggestions on how I can detect which computer is infected so I
can then try to figure out how to clean it?