"agent log off" when attemping to join a queue

Hi,

I have a single customer service queue with a number of agents logged in.

When a customer calls in and gets transfered to the queue, every now and then the call gets dropped with the agent log off message.

Has anyone seen anything like this before?

FreePBX 2.5.1.5
Queue module 2.5.4.8

Thanks,
Josh.