gigabit ethernet switch problem

[mobistar]

I am running a number of Slacko Puppy ( 5.3.1 ) workstations in an office environment. Each is connected to the backbone Netgear GS724 switch. Recently I needed an extra network port at one of the workstations ( a Shuttle PC with JMicron gigabit ethernet ). First I added a Netgear Prosafe 5 port gigabit switch. When Puppy booted it came up with no interface configured, when it had been working fine for months. The network setup wizard could not detect the network, and no port light was showing on the switch. I replaced the Netgear 5 port switch with an HP 5 port switch, which looked like it might have different internals, but the problem remained.
Plugging the ethernet cable directly into the PC allows the network interface to come up on the next reboot, so the config has not been lost.
Connecting the PC via the small switch lights up the activity LEDs on the switch and it stays on during a reboot, but goes off again at the point Puppy runs the rc script to bring the network up.
The command that is run in the rc.network script to check for the link heartbeat does not detect one and aborts the script.
Using ifcfg from the command line it is possible to assign a network address to eth0 and subsequently ping other addresses.

Has anyone any suggestions as to what the cause might be?