NetBSD 5.0.1 Networking Message

[Erik Winkler <ewinkler%erols.com@localhost>]

I am running NetBSD 5.0.1 (Generic PPC Kernel) on PowerMac 7600 with a  
powerpc 7455 processor upgrade card running at 1 Ghz.  I have 2 of the  
following ethernet cards installed (per NetBSD dmesg output):

3Com 3c905B-TX 10/100 Ethernet (rev. 0x30)

For some reason I keep getting the following messages and I don't see  
the cause:

ex1:  transmit underrun (9000) @20

The ex1 interface in this case is connected to a 100-BaseT switch as  
seen in the ifconfig command below:
ifconfig ex1
ex1: flags=8863<UP,BROADCAST,NOTRAILERS,RUNNING,SIMPLEX,MULTICAST> mtu  
1500
         
capabilities 
= 
3f00 
<IP4CSUM_Rx,IP4CSUM_Tx,TCP4CSUM_Rx,TCP4CSUM_Tx,UDP4CSUM_Rx,UDP4CSUM_Tx>
        enabled=0
        address: 00:10:5a:5d:e5:38
        media: Ethernet autoselect (100baseTX full-duplex)
        status: active
        inet 192.168.1.99 netmask 0xffffff00 broadcast 192.168.1.255
        inet6 fe80::210:5aff:fe5d:e538%ex1 prefixlen 64 scopeid 0x2


Thanks for any insight you can give regarding this error.

Erik