Internet Authentication Service — RADIUS

I currently have several Cisco switches and routers that are using RADIUS for administrative login. I also us e this same RADIUS server for VPN authentication for my firewall. This works great most of the time. However, and apparently for no reason, the RADIUS server stops allowing authentication of clients. Upon checking the server, everything appears to be in order. When I examine the event logs, however, I am getting the following error: A RADIUS message was received from the invalid RADIUS client IP address {ip address}. This has happened several times and seems to be a randomly occuring thing. I have found that if I open any IAS client record and reapply the settings, the problem goes away. Any thoughts?