100 base full duplex?

[0123456789]

What:
Dockstar (arch linux) on gigabit router
Mac mini (10.5.8) on 10/100 moca router

Background info:
I have been troubleshooting slow streaming from my Dockstar to my Mac. I found if I place my Dockstar on the other moca router I get better speeds. I occasionally need full gigabit speeds to the Dockstar from another Linux box. Instead of having to plug and unplug the Dockstar, I would like to see if can just leave the Dockstar on gigabit router and just toggle eth0 to 100 base full duplex or 1000 base full duplex. Currently it is set to the default auto negotiate.

What I have done:
I tried to use ethtool -s eth0 speed 100 duplex full, but that did not work.
If i try to turn off auto negotiate with ethtool I lose my network connectivity, even if I do it all on one line ethtool -s eth0 speed 100 duplex full autoneg off

Note: My router does not allow me to set port speeds.

Question:
Does anyone know what the syntax would be for ifconfig ( or another program) to do this?

Any information or suggestions would be much appreciated.

[WarheadsSE]

1) are they routers or switches?
2) If you get faster speeds from within one switchs versus traversal, then the uplink is causing the delay somehow.

Yeah, this isn't helpful to the specifics in the OP, sry.

I'd be more interested in why you can't just leave it on the GbE