but I don't have anything definitive. You may want to call in to support, frankly.
HOWEVER
When we looked at logs that looked a lot like this in the past, what it looked like was network connectivity issues with the DS. For instance, we know that networks will often sever the connection between the client and the DS without warning because it perceives the open port as a threat. Other kinds of network issues can have similar effects, whether it be routing problems, or a bad port on a switch, or things like that. You may want to verify that the clients are able to reliably stay connected, and that nothing on your network is causing issues.
Again, that's what the logs "look" like on initial inspection (that the client appears to have connectivity issues), but that's just an initial look, and you may need a deeper diagnosis. Check out the network for some "interesting" policies and then I guess let us know. :D
Nice suggestions btw ian. Wireshark is wonderful, and actually might point to the same things. :D