The problem seems to be that the latest 3c575_cb driver uses up too much PCI bus bandwidth. To improve performance, the driver instructs the 3Com card to poll the transmit ring for new packets to send, instead of explicitly telling the card when new packets have been queued. This reduces the load on the CPU, but the extra PCI bus traffic seems to cause congestion on some systems.
The 24-Aug-00 beta has a new module parameter for the 3c575_cb module, "down_poll_rate", that controls this polling feature. This can be set from 0 to 127; 0 disables polling, other values give the poll interval in units of 320 ns. The default is 20. On systems where polling causes problems, either increase the poll interval to the maximum of 127, or disable polling entirely.