Ethernet flow control???

HI.

We’ve been having lots of network disconnects and slow performance on our 40 mac clients.
Symptoms include start up so slow, that the client gives up looking for the OSX server for dir authentication.
They seem to be dropping packets like mad.

We’ve got d-link managed switches throught our net.
The 300 or so PC’s are working perfectly.

Enabling ethernet flow control at the end of the star on switch ports with macs has helped a lot, almost eliminating the problem. but is not ideal, ‘cos it seems to slow down pc’s on the same port. And requires very careful management of what computer is connected to which port

Can anyone suggest a reason for this?
Never used to be a problem….

l.