15 byte UDP at 20/sec, RCM3200 drops 1/24 on Win 7, 1/5600 on XP

I have 25 embedded systems which send and receive 20, 15 byte UDP datagrams per second to a PC through a crossover cable (no switch, no router). Inside a costate {}, the udp_sendto function always returns 15, never anything else. On an XP machine (for 8 years), on average, 1 in about 5600 packets get dropped, however, on a Windows 7 machine (now required), on average, 1 in 23 packets get dropped.Wireshark does not see the missed datagram, neither does the PC.
udp_open ( &socket, 6400, -1, 6500, datahandler )
I have tried udp_bypass_arp without success.
DC 9.62 with patches.
Any thoughts on how to get more of the packets in Win 7?

Have you tried a hub or network tap and a second PC with XP and Wireshark to see if that setup sees more of the packets on the wire than the Win 7 one? This might help to narrow down the issue.