Sorry Peter,
I can't help much more then this. It just really sounds like a networking problem. TCP is guaranteed delivery so the messages should be getting through. (Nothing says they won't be delayed) If the don't get through, the sender is notified and should retry.
At this point I would be asking my networks guru to break out wireshark/whatever is needed to verify that the network is working ok and running near the edge. Funny thing about ethernet is a bad cable in one section can cause all sorts of problems somewhere else on the other side of the network.