I have a box running Slackware 12.1, netgear gigabit card, Sempron 2200, 512mb ram, winfast k7s mobo. My switch shows that the nic is in GB mode. I'm copying some files to it, and performance is very slow - about 3mbps. I get occasional peaks of 50-60mbps, but these are real fast spikes. I'm getting a fairly constant 2-3 mbps.

So I put the network cable into the mobo 100mbps nic, set rc.init1.conf so that it will get an ip address from my dhcp server, reboot (is there a way to do this without rebooting?).

Resume file copy. Through this nic I get about 75mbps. Not bad for a 100mbps connection.

My question - how would I go about troubleshooting the poor performance via the gigabit nic?

Just a thought, do you have the appropriate alias assigned to the NIC? I have an Asus M2NPV-VM mobo, and the on-board NIC uses the alias forcedeth. In my /etc/modprobe.d/aliases I have:

alias eth0 forcedeth # on-board NIC in M2NPV-VM
# alias eth0 3c59x # NIC used in K6-III+ box
# alias eth0 tulip # NIC used in PII Deschutes box
# alias eth1 3c509 # NIC used in any box

The last three aliases are for other boxes I use around here. I keep them in the config file for easy reference.

Sure: /etc/rc.d/rc.inet1 restart