I would suggest that perhaps you are running out of tcp connections or you have a bad nic on your network.

If you check scoadmin / network / tcp of the NIC you should see how many tcp connections you are willing to allow. We set up most machines to be 512 but you can set it up any way you like.

I have seen several times, SCO 5.0.x boxes that the network seems to just die randomly and upon reboot then it works flawlessly. Usually it is one of two things, either there is a bad NIC on your network that is corrupting arp packet requests (wireshark or another packet sniffer should show you this symptom within a few minutes, and will usually be miss-labeled "Gratuitous Arp Requests" and should be called "non-conforming arp requests") or you are having something make more socket connections that you have tcp connections. As soon as you reach the number you specified, the NIC will stop processing all traffic.