Seems that the problem was "Allow the computer to turn off this device to save power" was enabled on the network interface. Since I disabled that, the machines have been running fine.

Regarding the DHCP issue, a couple years ago I had problems with FPW 2.6 clients losing connectivity to a DBF residing on a Windows 2003 network share, and after much head scratching figured out the connection was lost when the DHCP lease from a garden variety Linksys router expired. ... so with that lesson learned I use only static IPs now ...