My first thought is a bad connection or several bad connections somewhere in the network. But you say it is a different endpoint point from time to time. However, I would not rule out a bad connection. If you have this option, I would try and disconnect an endpoint one at a time and see if the problem continues. If it continues, there could be more than one problem connection. Therefore, I would divide and conquer by dividing the network in half or thirds.
I have not worked with xbees at all, but I have alot of experience with networking control equipment. In my experience, 9 times out of 10, it's hardware - especially using serial and coaxial cabling.
Hope this helps.