Found it. I checked `cat /proc/net/udp` and could see some packets being dropped in the UDP buffer queue; and whilst monitoring the dropped packets we're seeing from the clients, the numbers were going up in line with this. So I increased the `net.core.rmem` and `net.core.wmem` values a little on the server via sysctl, to increase the default UDP buffer size which has stopped the timeouts from occurring. I guess out the box the server has am appropriate value for the UDP buffer queue but when using FreeRADIUS it fills it up if there's enough traffic (and its all UDP) and then the kernel starts dropping packets. Whist I appreciate it's not FreeRADIUS related, is there any advice around the best sysctl values to use for a large FreeRADIUS server? Has anyone come across this before? Thanks J